The exoskeletal structures consist of dentine, or of a tissue resembling it, never of true bone. The orbits are marginal or lateral in position. With the exception of a caudal fin there are no median fins.
Fig. 313.—Restored outline of Lanarkia spinosa, in the position in which it occurs as a fossil, the head being flattened and the tail twisted round so as to appear in profile. On each side a much enlarged dermal denticle is shown. (From Traquair.)
Fam. 1. Coelolepidae.[[619]]—Head and anterior portion of the body flattened and expanded, with prominent lappet-like postero-lateral lobes, which may represent continuous lateral fin-folds or a very primitive type of pectoral fin. Nothing is known of the mouth, but it must have been ventral, nor of the position of the orbits. Branchial apertures unknown, but transverse markings on each side of the anterior part of Thelodus pagei may be indications of a branchial apparatus. The exoskeleton consists of a uniform covering of hollow pointed spines, devoid of a basal plate and open below (Lanarkia); or of minute shagreen-like tubercles (Thelodus). The tubercles or spines consist of dentine coated by ganoin. Of the only two known genera, Thelodus is a characteristic Upper Silurian genus from the Ludlow and Downtonian Beds of Lanarkshire. Detached scales are also known in the Upper Silurian of England. One species (Th. pagei) occurs in the Lower Old Red Sandstone of Forfarshire, and another (Th. tulensis) in the Upper Devonian of Russia. Lanarkia has only been found in the Downtonian Beds. None of the Coelolepidae exceed fourteen to fifteen inches in length.
Fig. 314.—Restored outline of the dorsal surface of Drepanaspis gemündenensis. The tail appears in profile. m.d, Median dorsal plate; p.l, postero-lateral plate; r, rostral plates. (From Traquair.)
Fam. 2. Drepanaspidae.—This family[[620]] affords an interesting transition to the more highly specialised and carapaced Pteraspidae. The head and anterior part of the trunk now form a broad oblong shield, rounded in front and abruptly marked off from the tail by conspicuous rounded angles. The exoskeleton is no longer uniform. In the caudal region the scattered spines or shagreen tubercles of the Coelolepids have become transformed into tuberculated quadrangular scales, which are further differentiated along the dorsal and ventral margins into ridge scales or fulcra; and from a similar source by a process of basal fusion a series of larger or smaller dermal plates are formed as components of large dorsal or ventral shields. The dorsal shield (Fig. 314) is formed by a large central plate; the postero-lateral portions by two narrow falciform plates; and the anterior margin by a series of smaller rostral plates. Between the larger plates the shield is completed by numerous small polygonal plates. All the plates are superficially ornamented by small stellate tubercles. The ventral armature (Fig. 315) is similar to the dorsal. A large mental plate forms the hinder margin of the transverse slit-like mouth, the anterior limit of which is defined by the rostral plates already mentioned. Laterally may be seen a pair of small plates (x), each perforated by a small aperture, and probably indicating the position of some kind of sense-organ. Posteriorly there is a large median ventral plate, in relation with a pair of anterior and a pair of posterior ventral plates. The areas between the larger plates are filled in by numerous small polygonal plates. It is possible that there is a single external branchial aperture on each side, near the postero-lateral angle of the shield and behind the posterior ventro-lateral plate. The sole representative of the family is Drepanaspis gemündenensis, from the Lower Devonian of Gemünden in Rhenish Prussia. Large examples of this fossil must have exceeded two feet in length.
Fig. 315.—Ventral surface of Drepanaspis (tail in profile). a.v.l, Anterior ventro-lateral plates; e.l, external lateral; m, mental plate; m.v, mid-ventral; p.l, postero-lateral; p.v.l, posterior ventro-lateral; r, rostral; x, orbit or sensory plate. The mouth and the supposed cloacal aperture are indicated in black. (From Traquair.)
Fam. 3. Psammosteidae.—To this family are referred certain dermal plates occurring, in a more or less fragmentary condition, in the Old Red Sandstone and Devonian formations of Great Britain and Russia. In their size and shape, and in their stellate tubercles, these have been compared to the dorsal, postero-lateral, and ventral plates of Drepanaspis. That Psammosteus is closely allied to Drepanaspis seems certain, but for the present the two genera may be retained in separate families.