Fig. [141].—A, Hypostome of Bronteus polyactin, Ang. showing maculae, × 4. B, Left macula of Bronteus irradians, Lindst. × 12. (After Lindström.)
The “labrum” or “hypostome” is attached to the doublure in front (Fig. [137], B, a); it is commonly an oval or shield-shaped plate, but is occasionally nearly square. Its surface is sometimes divided into two or three areas by shallow transverse grooves (Fig. [141], A), Just behind the middle of the hypostome, or when transverse grooves are present either in or near the anterior groove, there are often found a pair of small patches or “maculae” which are more or less oval or elliptical in outline (Fig. [141]). The maculae may be (1) surrounded by a raised border, or (2) in the form of pits, or (3) raised like tubercles. In some cases the entire surface of a macula is smooth and glossy; in others either the whole or a part is covered with granules, and in the latter case the granules may be limited to the internal third (Fig. [141], B) or to the central portion. Sections of a macula show that the granules are really globular lenses similar to those of the compound eyes on the dorsal surface of the head. Some of the maculae which are without lenses show no structure, but in others there is a spongy or irregularly polyhedric structure with prisms, resembling the marginal zone of the prismatic eyes of some genera. There seems no doubt that the maculae with lenses are visual organs, and those without are degenerate eyes. They occur in some genera which, according to Lindström, are without eyes on the dorsal surface. Maculae do not appear to be present in other Crustacea, but they have been compared with a median organ, found just in front of the hypostome in Branchipus.[[189]] Maculae, have so far been found in 136 species of Trilobites belonging to 39 genera ranging from Lower Cambrian to Carboniferous.
A “metastoma” or lower lip plate (Fig. [142], Ep) is found just behind the hypostome in Triarthrus, but has not been noticed in any other genus. Between the hypostome and the metastoma lies the mouth.
The segments of the thorax are free, and their number varies from two in Agnostus (Fig. [146]) to twenty-six in Harpes (Fig. [150], A). In the Trilobites confined to the Cambrian period the number (except in the Agnostidae) is usually larger than in the genera found in the Ordovician and later periods. Owing to the free thoracic segments many Trilobites were able to curl up somewhat after the manner of a Wood-louse (Figs. [137], D, 138). The axial part of each thoracic segment is more or less considerably arched. Usually it consists of three parts: (i.) the largest part (Fig. [137], C, a), called the ring, is band-like in form, and is always visible whether the Trilobite is extended or coiled up; (ii.) in front of the ring is a depressed, groove-like part (Fig. [137], C, b) separating it from (iii.) the articular portion (c) which is convex in front and extends beneath the ring of the preceding segment; this part is only visible when the Trilobite is coiled up or when the segments are separated. In some few genera the axial part consists of a simple arched band without either a groove or a specially modified articular portion. The pleurae (Fig. [137], A, l, C, d-f) are fixed firmly to the axis, and have the form of narrow bands with the ends rounded, obtuse, pointed, or spinose. In a few cases the pleurae have a plain surface; but usually they possess either a ridge or a groove (Fig. [137], C, g); the former is generally parallel to the margins of the pleura, the latter is generally oblique, being inclined backwards from the axis. Sometimes in front of the ridge there is a small groove. On the ventral surface each pleura shows, at its outer extremity, a reflexed margin or doublure. At some distance from the axis the pleurae are bent downwards and backwards. The point where this bend occurs is called the “fulcrum” (e); it divides the pleura into an internal and an external part: the internal part (d-e) is flat or slightly convex, and just touches the front and back margins of the adjacent pleurae; the external part (e-f) may be (i.) narrower than the internal part, so that it is separated from the previous and succeeding pleurae; such occurs principally in pleurae with ridges, as in Cheirurus and Bronteus; or (ii.) it may be in the form of a long cylindrical process, as in many species of Acidaspis; or (iii.) the external part may be of the same width, either throughout or in part, as the internal part, and may overlap the next pleura behind; this type is found principally in pleurae with a groove such as in Phacops, Calymene, Sao, Asaphus, Ellipsocephalus.
In some Trilobites there is beyond the fulcrum a smooth, flat, triangular part at the front margin of the pleura; this part is known as the “facet,” and forms a surface articulating with the preceding segment which overlaps it.
In the remarkable form Deiphon (Fig. [151], E) the pleurae are separate throughout their entire length.
In some Trilobites broad and narrow forms of the same species occur—the difference being seen especially in the axis. The former are regarded as females, the latter as males.[[190]]
The segments of the abdomen or pygidium (Fig. [137], A, 3) are similar to those of the thorax, except that they are fused together. In a few forms, such as Illaenus (Fig. [150], F) and Bumastus, the fusion is so complete that no trace of segmentation can be seen on the dorsal surface. Usually, however, the segments are easily distinguishable; the number seen on the axis is commonly greater than on the lateral parts of the pygidium; this difference is particularly well shown in Encrinurus. In Trilobites which have grooved pleurae the conspicuous grooves seen on the lateral parts of the pygidium are the grooves of the pleurae, the sutures between the pleurae being less distinct. The shape of the pygidium may be semicircular, a segment of a circle, trapezoidal, triangular, semi-parabolic, etc.; its size varies considerably; in the Cambrian forms it is usually small, but in the Trilobites of later periods it becomes relatively larger. The number of segments in the pygidium varies from two to twenty-eight. The axis of the pygidium tapers more rapidly than that of the thorax; sometimes it reaches quite to the posterior end of the body, but is commonly shorter than the pygidium; in Bronteus it is extremely short, and the grooves on the lateral parts of the pygidium radiate from it in a fan-like manner. Occasionally, as in Bumastus, the axis cannot be distinguished from the lateral parts. In a few early Trilobites (Olenellus, Holmia, Fig. [148], Paradoxides, Fig. [147]) the lateral parts of the pygidium are very small. In some genera, such as Asaphus, the marginal part of the pygidium forms a flattened or concave border. The margin may be entire or produced into spines, and sometimes (Fig. [151], C) a caudal spine comes off from the end of the axis. On the ventral surface of the pygidium there is a marginal rim similar to the doublure of the cephalic shield. The anus is on the ventral surface of the last segment of the pygidium.
Although Trilobites are often found in abundance and in an excellent state of preservation, it is only in very rare cases that anything is seen of the ventral surface except the hypostome and the reflexed borders of the cephalic shield, of the thoracic segments, and of the pygidium. The usual absence of appendages is probably due to their tenuity. Billings, in 1870, first obtained clear evidence of the presence of pairs of appendages, in Asaphus platycephalus. Soon afterwards Walcott[[191]] showed their existence in American specimens of Asaphus megistos, Calymene senaria, and Cheirurus pleurexacanthus. In the two latter species the appendages were found by cutting sections of curled-up specimens obtained from the Trenton Limestone; 2200 examples were sliced, of which 270 showed evidence of the existence of appendages. They were seen to be present on the head, thorax, and pygidium; a ventral uncalcified cuticle with transverse arches was also found. By means of sections of curled-up specimens it was difficult to determine satisfactorily the form and position of the appendages. Subsequently extended specimens of Triarthrus (Fig. [142]) and Trinucleus, showing the ventral surface and appendages clearly, were discovered in the Utica Slate (Ordovician) near Rome, New York. A full account of the appendages in those specimens has been given by Beecher.[[192]]
