Fig. 320.—Restored outline of Birkenia elegans Traq., one-half larger than natural size. b.a, Branchial aperture; d, dorsal fin. (From Traquair.)
Fig. 321.—Restored outline of Lasanius problematicus, enlarged, r, Post-cephalic rods; r′, row of small spine-like scutes; v.s, mid-ventral spine-like scales. (From Traquair.)
III. Antiarchi.
The organisms comprising this group[[630]] resemble the Ostracodermi in possessing a well-developed carapace of bony plates and a heterocercal tail, as well as in many of the purely negative features which are characteristic of the latter group.
Fig. 322.—Restored outline of Pterichthys milleri. The upper figure represents a dorsal view, and the lower a lateral view. The dotted lines indicate the course of the lateral line system. a.d.l, Antero-dorso-lateral; ag, angular; a.m.d, anterior median dorsal; a.v.l, anterior ventro-lateral; e.l, extra-lateral or operculum; l, lateral; l.occ, lateral occipital; m, median or interorbital plate; m.occ, median occipital; o, orbit; p.d.l, posterior dorso-lateral; p.m, pre-median; p.m.d, posterior median dorsal; pt.m, post-median; p.v.l, posterior ventro-lateral. ——, Plates investing the limbs: c, central; d.a, dorsal anconeal; d.ar, dorsal articular; e.m, external marginal; i.m, internal marginal; m.m, marginals; t, terminal. (From Traquair.)
The remarkable dorsal shield is divided into a small cephalic portion and a much larger hinder part investing the greater part of the trunk, both of which are strongly arched above and flattened ventrally, with a movable articulation between the two. The cephalic shield is formed by numerous symmetrically-disposed tuberculated plates, suturally connected with one another, and, like the other exoskeletal structures, containing bone lacunae (Fig. 322).[[631]] The orbits are close together, near the middle of the dorsal surface, and between them there is a small median interorbital plate, with a deep pit on its inner surface, possibly for a parietal organ. A small lateral plate (e.l.), evidently free behind, suggests the presence of an operculum. Nothing is certainly known about the jaws or the nostrils. The mouth is situated just behind the anterior margin of the cephalic shield on the ventral surface, and in front of it there are two plates, which in Bothriolepis canadensis have their oral margins fringed by small "denticles"; it is possible that these plates represent the components of a secondary upper jaw. The dorsal armature of the trunk is shown in Fig. 322. Ventrally it is completed by a pair of anterior ventro-lateral plates and a pair of posterior ventro-lateral plates with a small median plate between the two pairs. Articulating with the anterior ventro-lateral plates by means of a complex hinge joint there is a pair of pectoral appendages of a kind entirely without parallel in any other vertebrated animals. Each appendage is completely encased by numerous suturally connected plates, and about the middle of its length there is a second movable joint. The appendages are hollow, and their cavities probably contained the muscles by which the limbs were moved, and the blood-vessels and nerves for their nutrition and innervation. A lateral line system of the normal type is present in Pterichthys, consisting of a lateral groove along the side of the trunk, and of supra-orbital and infra-orbital grooves, and post-temporal and infra-orbital commissures, on the head. The free portion of the body and the tail are invested by imbricated and finely tuberculated scales, which form fulcra in front of and behind the small dorsal fin. There are no pelvic fins. The caudal fin is heterocercal.
Fam. 1. Asterolepidae.—The best known genera are Pterichthys from the Lower Old Red Sandstone of Scotland and the Devonian of Eifel, and Bothriolepis, a more widely distributed genus which occurs in the Upper Old Red of Scotland and Shropshire, and in the Upper Devonian of Russia and Canada. Two other genera, Asterolepis and Microbrachius, are also found in the Old Red Sandstone of Scotland.[[632]]