8 AGNATHA. 



and a large median dorsal spine (s) posteriorly; a pair of orbital plates 

 occurs, one completely surrounding the orbit (o) on either side and 

 partially inserted between the rostrum and central disc; and there is a 

 pair of postero-lateral cornua (c), each pierced with a large (probably 

 branchial) cleft or foramen. The typical species is Pteraspis rostrata, 

 from the Lower Old Red Sandstone of Herefordshire, Monmouthshire, 

 and Glamorganshire, and its shield sometimes measures about 0'14 m. in 

 length. A single fragment in the British Museum shows parts of the 

 dorsal and ventral shields in natural association, with a few small rhom- 

 boidal scales of the caudal region, which slightly overlap each other and 

 are ornamented with few, delicate, imbricating ridges, parallel to the 

 overlapped anterior margins. One specimen of a second species, P. 

 crouehi, from the same formation, is important as showing the sensory 

 canals infiltrated with manganese (fig. 5). The genus is also common in 

 the Lower Devonian of Galicia, and species are recorded from Forfarshire 

 and the Eifel. 



ORDER 2. OSTEOSTRACI. * 



The dermal shield in this order also comprises three layers, 

 but the middle vascular zone does not consist of cahcellse or 

 spaces, being traversed instead by an abundant network of 

 blood-vessels which subdivide the hard tissue into large poly- 

 gonal areas in which no such vessels are conspicuous. The 

 calcification both of this and the dense inner layer exhibits 

 many bone-cells ; the outer layer, which develops a tubercular 

 ornament, consists of vascular dentine. There are no traces of 

 dermal sense-organs either upon or within the shield. The 

 dorsal shield consists of one principal piece, in which the orbits 

 are placed close together mesially ; but the vacuity between 

 these, as also another median space immediately behind them, is 

 closed by a separately-calcified small plate. There is also often 

 a supplementary plate behind, which seems to consist of fused 

 body-scales. A ventral plate opposes at least the latter. 

 There are no paired appendages, except those apparently 

 connected with respiration. 



The Cephalaspidae are the best-known family of Osteostraci, 

 having the trunk completely covered with overlapping scales, 

 which are much deepened on the flank. The dorsal shield is 

 rounded or tapering in front, abruptly truncated behind, and 

 the interorbital piece is firmly fixed. The median fins are small. 



