Fam. 2. Cephalaspidae.[[623]]—In this family the dorsal shield is rounded in front, strongly arched above, with its postero-lateral angles produced into highly characteristic cornua (Fig. 318). The shield consists of a single piece, but as the outer surface is ornamented by small tubercles arranged in polygonal areas, it is probable that it has been formed by the basal fusion of numerous primitively distinct polygonal plates (Fig. 319, A). Between the orbits there is a separately calcified but fixed plate, which bears a hollow prominence, probably for the reception of a parietal organ. In some genera certain of the anterior dorsal and ventral scales of the trunk fuse into a continuous plate. Internally to the postero-lateral cornua the middle layer of the shield is prolonged backwards into a pair of singular flap-like lobes, which have been variously interpreted as corresponding to the lateral lobes of the Coelolepidae, to pectoral fins, or to opercula. The scales of the trunk and tail are rhombic and imbricated; on the sides of the body they are remarkably high and narrow.
Fig. 319.—The dorsal shield of Cephalaspis lyelli (A), and an outline sketch of the dorsal shield of Eukeraspis pustulifera (B). c, Postero-lateral cornu; d, posterior angle; i.p, interorbital prominence; o, orbit; o.p, orbital prominence; p.s, posterior spine; p.v, postorbital valley. (From Lankester.)
The best known genus is Cephalaspis. The earliest remains are found in the Ludlow Tilestones. The genus is also represented in the Ledbury Passage Beds, the Lower Old Red Sandstone of Scotland, and the Upper and Lower Devonian of Canada. Most of the species are of small size, but C. magnifica,[[624]] from the Caithness Flagstones, the largest of all the Cephalaspids, has a shield 8½ inches long, and 12 inches across the widest part. Auchenaspis occurs in the Ludlow Tilestones and the Ledbury Passage Beds, and also in the Upper Silurian of the Isle of Oesel in the Baltic. Another genus, Didymaspis, has been found in the Lower Old Red Sandstone of Ledbury.
Fam. 3. Tremataspidae.—The interorbital plate is free, and hence it is often lost in the fossils. Several species of Tremataspis occur in the Upper Silurian of the Isle of Oesel.
As regards the origin and mutual relationships of the different families comprising the Heterostraci, it has been urged with great force by Dr. Traquair[[625]] that they constitute a natural sequence of forms, beginning with organisms whose Elasmobranch ancestry is extremely probable, and leading to highly-specialised types, which, considered by themselves, possess little to justify any conclusions whatever as to their origin or kinship. The Coelolepidae form the starting-point, and in the light of their exoskeleton of dermal denticles, their derivation from some primitive Elasmobranch prototype seems a reasonable inference.[[626]] From the Coelolepids the path of specialisation through the Drepanaspidae and Psammosteidae to the Pteraspidae is marked (i.) by the basal concrescence of isolated denticles to form, first, numerous small polygonal plates, and then larger and less numerous plates, as the constituent elements of a characteristic dorsal shield, leaving, however, the denticles of the rest of the body to become converted into a rhombic squamation; (ii.) by modifications in the "lateral fin-lobes," which may become enclosed in the developing dermal armour (e.g. Drepanaspis), or cease to be recognisable (e.g. Pteraspis). The affinities of the Osteostraci are very obscure, and their inclusion with the Heterostraci in the same group (Ostracodermi) has hitherto rested mainly on such negative evidence as the supposed absence of paired limbs, jaws, and teeth; in fact, it has been affirmed that "there is absolutely no reason for regarding Cephalaspis as allied to Pteraspis beyond that the two genera occur in the same rocks."[[627]] It is possible, however, that in Ateleaspis we have an annectent form, which in some measure combines the structural peculiarities of the two groups. That this singular genus belongs to the Osteostraci is proved by the presence of bone lacunae in its dermal hard parts, a conclusion which is strengthened by the apparently dorsal position of the orbits and the presence of a dorsal fin. On the other hand, its close resemblance to the Coelolepids in the general contour of its laterally-lobed body, and the probability that its mosaic and tuberculated head-shield has been formed by the concrescence of Coelolepid denticles, is at least significant of a relationship to the more primitive Heterostraci. Little can be conjectured as to the habits of these ancient "Fishes." The form and regional proportions of the body, which in some respects often remind one of organisms so diverse as a King Crab, or a Loricaroid Teleost (such as Liposarcus), are strongly suggestive of a grovelling, bottom-feeding, sluggish habit of life, in sharp contrast to the more active and predaceous Fishes whose appearance is coincident with the extinction of the Ostracodermi at the close of the Devonian period. Habits such as these may well be associated with much structural degeneration, even, it may be, with the loss of paired fins, and hence it is not altogether improbable that the Ostracodermi are outcasts from the Elasmobranchs, a degenerate race which has sought safety in a sequestered life and a coat of mail.
Order III. Anaspida.
This group has been instituted by Traquair[[628]] for the provisional reception of two remarkable genera, which, owing to the absence of precise knowledge of the histology of their exoskeletal structures, cannot at present be referred either to the Heterostraci or the Osteostraci, and for which, as their discoverer remarks, no place can be found in the system unless they are admitted to the Ostracodermi.
Fam. 1. Birkeniidae.—Body fusiform and fish-like. Head bluntly rounded, without a cranial shield. Caudal fin bilobate and heterocercal A median row of scales with recurved spines arranged along the ventral surface. Orbits, jaws, teeth, paired fins, and endoskeleton unknown.
In Birkenia (Fig. 320) the body is invested by longitudinal rows of narrow scales arranged in oblique transverse rows, which are replaced on the head by much smaller, peculiarly disposed, spindle-shaped scutes. On the side of the hinder part of the head there is an oblique row of small apertures, possibly branchial. A small remote dorsal fin, invested by the trunk scales, is present. Birkenia elegans, the only species known, does not exceed 3½ inches in length. Less is known about the second genus, Lasanius, of which there are two species. Except for the mid-ventral series of spiny scutes, and a row of slender, parallel, rod-like structures, the body appears to have been naked (Fig. 321). The two genera belong to the remarkable series of fossil Fishes from the Silurian rocks of Lanarkshire. Rare in the Ludlow series, Birkenia is by far the most common of the Fishes of the over-lying Downtonian Beds. Lasanius is confined to the latter horizon. Euphanerops, from the Upper Devonian of Canada, is probably related to this family, but lateral branchial apertures are not known.[[629]]