The basal plate of the cephalis, or the "cortinar plate," the "Basal-Scheibe" of Bütschli, has constantly, according to this author, two pairs of pores, an anterior smaller pair of jugular pores (the pores I of Bütschli) and a posterior larger pair of cardinal pores (the pores II in his description) (compare Pl. [95], figs. 3, 6). This applies to the majority of Spyroidea, but by no means to the whole group. We find basal plates with two pores only (Pl. [95], fig. 1), with three pores (figs. 2, 5), with six pores (figs. 4, 7), with nine pores (Pl. [87], fig. 2), &c. The forms with two basal pores may be derived directly from Semantis (Pl. [92], figs. 1, 2); the common forms with four pores from Semantrum (figs. 3-5); the rarer forms with six pores from Semantidium (figs. 6, 7); and the forms with three pores from Cortiniscus, &c. In this as well as in other respects the variety of different forms and of developmental variations is far greater than Bütschli (1882, loc. cit.) supposed. The bars between the pores of the basal plate possess the same value and the same names as in the Semantida (compare above, p. [954]).
The coryphal plate of the cephalis, its upper or apical lattice-plate, does not exhibit such important differences as the opposite basal plate, is far less variable, and is usually pierced by numerous, smaller pores. Some larger pores lie, often in pairs, on the right and left side of the sagittal constriction, and have in some groups a regular form and disposition. Either in the middle of the coryphal plate, or (usually) nearer to its dorsal margin, there arises in majority of Spyroidea an apical horn, directed either vertically or more or less obliquely backwards. In many Spyroidea three horns are developed in the coryphal plate, the odd middle apical horn and two paired frontal or lateral horns arising on each side of the latter and directed more forwards (Pl. [84], figs. 9-12). Sometimes the apical horn disappears, while the two frontal horns remain (Pl. [95], fig. 12).
The anterior or ventral plate of the cephalis (the "Hinterseite" of Bütschli), and the posterior or dorsal side (the "Vorderseite" of that author), exhibit in the majority of Spyroidea more or less marked differences in the number, form, and disposition of their pores, which require a far more accurate description than is here possible. Usually the sagittal constriction of these two plates, produced by the primary vertical ring, is deeper and sharper in the middle of the ventral than in that of the dorsal plate. On both sides of the ring there appear in each plate usually two or three pairs of larger pores, whilst numerous smaller pores are situated towards the lateral sides. We may distinguish the large pores of the ventral plate as facial pores (upper orbital, middle nasal, and lower maxillary pores), and the opposite large pores of the dorsal plate as occipital pores (upper epoccipital, middle mesoccipital, and lower suboccipital pores). A closer comparison of these pores, and of the separating bars in the numerous Spyroidea, may show a regularity of development similar to that offered by the homologies of the parts in the skeleton of the Echinodermata, or of the bones in the skull of the Vertebrata.
The two convex lateral plates of the cephalis, or the right and left sides, both symmetrically equal, do not exhibit the same regularity in the shape, number, and disposition of the pores that the four other sides do. Usually their pores are much smaller and more numerous. In very few forms only a distinct frontal ring is visible (corresponding to that of the Coronida), and in this case the lateral pores are sometimes disposed symmetrically on both sides of this ring. These Spyroidea may have arisen directly from corresponding forms of Coronida.
The sagittal ring (or the primary vertical ring) inherited from the Stephanida and Semantida, shows in the Spyroidea great variety in its form and in its relation to the cephalis. These variations are far greater than Bütschli (1882, loc. cit.) supposed. We may distinguish the following six principal cases:—A. The ring lies completely in the wall of the cephalis, and causes a deep sagittal constriction in it. In this case the lower part or basal segment of the ring separates the basal pores into pairs; its anterior part or ventral rod the facial pores; its upper part or coryphal rod the apical pores; and its posterior part or dorsal rod the occipital pores. No part of the ring is free in the shell-cavity. B. The greater part of the ring lies enclosed in the shell-wall; only its dorsal rod arises free in the shell-cavity and ascends vertically or obliquely to the apex, where it is usually prolonged into the apical horn. This seems to be the most common case. C. The coryphal and the basal part of the ring lie enclosed in the shell-wall, with its dorsal and ventral rod free in the shell-cavity; the dorsal rod ascends vertically to the apex, the ventral rod obliquely upwards to the upper part of the facial plate (the nasal or orbital region). This case, regarded by Bütschli as the usual one, is far less common than he supposed. D. The greater part of the ring lies free in the shell-cavity, its basal rod only is enclosed in the shell-wall, and separates the right from the left group of the cortinar pores. This case seems to be rarer than the preceding. E. The whole ring lies free in the shell-cavity, and is connected with the sagittal constriction of the shell-wall by numerous short radial beams. The distance of the shell-wall from the enclosed ring is usually least on the basal rod, and greatest on the dorsal rod. F. The whole ring lies free in the shell-cavity (as in the preceding case), and is connected by numerous short radial beams with a secondary larger, concentric sagittal ring, which is developed in the longitudinal constriction of the shell-wall.
Comparing these six principal cases, in respect of the relation of the primary sagittal ring to the cephalis of the Spyroidea, we may suppose that they represent together a continuous phylogenetical series of which the first (A) is the original, and the last (F) the latest case; and that the true cause of the various changes is a successive separation of the ring, which becomes more and more free and independent from the lattice-plate of the cephalis. Since the intimate study of the structure of the cephalis in many Spyroidea is very difficult, it requires further accurate observations (from all six sides of the shell). There are also some other more difficult complications of its structure, which cannot be solved without exact study and extended comparison.
Of peculiar importance for the differentiation of the numerous genera of the Spyroidea are the basal feet or the descending and diverging apophyses, which are developed from the base of their cephalis. In general they exhibit the same typical shape which we find in the Plectellaria as well as in the Cyrtoidea, and which we regard therefore as an important common character of the majority of Nassellaria. We may therefore distinguish here also the three primary, and the other ones as secondary feet. The three primary feet, or the "cortinar feet," are the same that we found in the Plagonida and Plectanida, in Cortina and Cortiniscus among the Stephoidea, and in the majority of Cyrtoidea. They also reappear in the same characteristic disposition and connection with the basal plate of the cephalis, in the majority of Spyroidea. The odd caudal foot (c) is the lower prolongation of the dorsal rod of the sagittal ring, which is prolonged upwards into the apical horn. The two paired pectoral feet, however (p′, p′′), are the descending prolongations of the coracal rods (e) which separate the jugular pores (i) from the cardinal pores (k) of the cortinar plate (compare Pl. [95], figs. 2-6).
The three cortinar feet are nearly equally developed in the majority of Spyroidea (compare Pls. [84], [89]). But often the odd caudal foot exhibits a different shape from that of the two paired pectoral feet, and in many genera of the group it becomes more or less rudimentary. It has quite disappeared in the subfamily Dipospyrida (Pl. [85]); here the two pectoral feet only are developed (often excessively), and are usually opposite in the frontal plane. Such dipodal forms, very common in the Spyroidea, are never found in the Cyrtoidea.
The secondary feet, which we contrast with the three primary feet as later productions, exhibit great variations in number and development. Very frequently three interradial secondary feet become developed, alternating with the three primary cortinar or perradial feet, and opposed to them in pairs (Pl. [95], figs. 7, 8). In these hexapodal Spyroidea (or Hexaspyrida) an odd anterior or sternal foot (z) is opposite to the odd caudal foot, and two paired posterior or tergal feet (t′, t′′) to the two pectoral feet. When the sternal foot becomes rudimentary or lost, pentapodal forms arise (Pentaspyrida, Pl. [95], figs. 9-11), and when the two odd feet (caudal and sternal) disappear, tetrapodal forms arise with two pairs of feet (Therospyrida, Pl. [89], figs. 5, 6). The latter differ from those quadrupedal forms in which two opposite sagittal feet (the caudal and sternal) alternate in the form of a cross with two paired lateral or pectoral feet (Tetraspyrida, Pl. [53], figs. 19, 20). In many Spyroidea the number of basal feet is greatly increased, and they form a regular radial corona around the basal plate, like a circle of tentacles (Polyspyrida, Pl. [87]). In this latter case the feet are usually flat and lamellar, whilst in the other groups they exhibit a very variable form, as is seen in Pls. [83]-[89].
The Central Capsule of the Spyroidea has been accurately observed in only a few genera, and requires a further exact examination regarding the different modifications which occur in their different families. It is very probable that these will agree with the well known modifications in the corresponding groups of Cyrtoidea. The characteristic Monopylean structure of the capsule (with porochora and podoconus) was first described by Richard Hertwig in his Ceratospyris acuminata (loc. cit., p. 72, taf. vii. fig. 2). I found the same afterwards in many other forms. In the Zygospyrida and Tholospyrida the central capsule seems to be usually bilobate (bisected by the sagittal ring), and enclosed in the two chambers of the cephalis. The two lobes of the capsule (right and left) are equal and connected by a smaller middle part, which contains the transverse elliptical nucleus and is enclosed by the sagittal ring; often each lobe contains a large oil-globule. In some forms, however, three or four lobes (sometimes perhaps more) are developed, which pierce the cortinar pores of the basal plate and depend freely between the basal feet (Pl. [53], fig. 19). In the Phormospyrida and Androspyrida the formation of such basal lobes seems to be more frequent; they are here enclosed by the thorax. Usually each lobe contains a large oil-globule. In Nephrospyris (Pl. [90]) and in some similar genera the central capsule is violin-shaped, deeply bisected by the sagittal ring; in each of the two lobes a series of oil-vesicles is developed at both poles of the transverse axis (figs. 7, 10); the nucleus, a transverse cylindrical body, is placed in the latter. In the periphery of the voluminous calymma of this singular genus a large number of globular cells are developed (figs. 7, 10), probably symbiotic xanthellæ. In some forms of Nephrospyris these xanthellæ are enclosed by a peculiar inflated girdle, developed in the periphery of the kidney-shaped skeleton (Pl. [90], figs. 1, 4-6).