OStRACODERJIi. 



2G1 



referred to various classes of animals, including Crustacea and Arachnida. 

 They were first recognized as fishes by L. Agassiz, but their systematic 

 position within that group is still quite uncertain, and it may well be that 

 they are not fishes at aJl. They are confined to the upper Silurian and 

 to the Devonian. Their internal skeleton is very little known. They are 

 without any trace of jaws, visceral arches, paired fins and their girdles, 

 and a segmented axial skeleton. TTie head and anterior part of the 

 trimk are alwaj-s invested by a powerful dermal armour, consisting of 

 large plates, of which one covers the whole dorsal aspect of the head. 

 The orbits are small and frequently placed near together on the dorsal 

 surface of the cephalic shield as in Limulus. They are divided into three 

 groups. 



'■/■ -, 



p. 0. V, 



Fia. 140. — Pteraspis rostrata, restored, J, side vibw (from Smith Woodward). 



Heterostraci. Head and anterior part of the body covered by a 

 dorsal shield, rarely simple, usually composed of several pieces. The 

 orbits are small and placed far apart on the outer edge of the dorsal shield. 

 Hinder part of body covered by rhombic plates. The cephalic plates are 

 without bone cells. Pteraspis Kner (Fig. 140), dorsal shield narrow in 

 front and prolonged into a rostrum, with a median spine behind ; 

 Palaeaspis Claypole, Cyathaspis Lank. 



2 Osteostraci. Head 

 covered by a large 

 dorsal shield on which 

 the two orbits lie near 

 together (Fig. 141) and 

 which contains bone 

 cells ; body covered 

 with rhombic scales. 

 Cephalnspis Ag. (Fig. 

 141), the posterolateral 

 angles of the shield are 

 produced and there is a 

 median spine, dorsal 

 and anal fins with well- 

 developed rays, upper 

 Silurian and Devonian. 

 Tremataspis Schmidt, 

 upper Siktrian. 



3. Antiarcha. Head 

 and body covered by 

 an armour of symme- 

 trically arranged bony 

 plates. Orbits close 



Quart. J. of Geol. Soc, 1857, 13, p. 48. Huxley, Cephalaspis and Pteraspis, 

 Ibid, 12, 1856, p. 100 ; 1858, 14, p. 267 ; 1861, 17, p. 163. Lankester and 

 Powrie, Cephalaspidae, Palaeont. Soc, 1868. 



«. y. 



».c. 



p.9i 



Fig. 141. — Cephalic shield of Cephalaspis Agassizi (after 

 Lankester, from Woodward), x i, L. Old Ked Sandstone, 

 n/, antorbital fossae ; ap antorbital prominences ; ^ inter- 

 orbital ridge ; mc, marginal cells ; or orbital rim ; va 

 posterior angle; pc posterior comu ; pov postorbital 

 depression ; pr posterior ridge ; ps. posterior spine ; r rim. 



