OSTRACODERMI. 



2G1 



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

 Thoy were first recognized as fishes by L. Agassiz, bvit their systematic 

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

 they are not dshes at all. They are confined to the upper Silurian and 

 to the Devonian. Their internal skeleton is very little kno\vn. They are 

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

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

 trtrnk are always 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. 



«•/• - 



Pig. 140. — Pteraspis rostrata, restored, J, side view (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 

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

 Palaenspis Claypole, Cjjathaspis 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. 

 Cephalaspis Ag. (Fig. 

 141), the posterolateral 

 angles of the sliield are 

 pi'oduced and there is a 

 median spine, dorsal 

 and anal fins with well- 

 developed rays, vipper 

 Silurian and Devonian. 

 Tretnataspis Sch i idt, 

 upper Silurian. 



3. Antiarcha. Head 

 and body covered by 

 an armour of symme- 

 trically arranged bony 

 plates. Orbits close 



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

 lankester, from Woodward), x J, L. Old Bed Sandstone. 

 af, antorbital fossae ; ap antorbital prominences ; ig inter- 

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

 posterior angle ; pc posterior cornu ; pov postorbital 

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



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 

 PowTie, Cephalaspidae, Palaeont. Soc, 1868. 



