﻿INTRODUCTION. XIX 



cases be readily distinguished. All the dermal armour of the 

 Ostracoderms is characterized by an extraordinary development of 

 vascular sinuses or channels in the middle layer, while that of 

 Coccosteus and its allies consists of nearly homogeneous dense bone 

 with only a slightly cancellated structure in its thicker portions. 



The marked affinity between the Heterostraci and Osteostraci has 

 already been demonstrated by Huxley x and Lankester 2 ; and all 

 the recent observations detailed in the following Catalogue tend to 

 confirm the general results of that demonstration. It is, however, 

 necessary to add a few remarks on the relationship now perceived 

 between the Antiarcha (i. e., the family Asterolepidse) and the 

 Osteostraci; more especially as these have not hitherto been 

 enumerated, and Cope's statement on the subject is made with 

 hesitation. The comparatively specialized genera Auchenaspis, 

 Didymaspis, and Tremataspis may first be compared with 

 Pterichthys in the arrangement of the dermal armour. In each case 

 the head exhibits only a dorsal shield, while the abdominal region 

 is covered both dorsally and ventrally by an armature that meets in 

 a close suture laterally. As clearly shown in Tremataspis, and less 

 distinctly observed in the other Osteostraci just mentioned, the 

 ventral shield terminates abruptly in front, as in Pterichthys ; and 

 the only fundamental difference between the specialized Osteo- 

 stracan and the ordinary Antiarchan type seems to be that the 

 armature of the former consists of few p]ates, while these are sub- 

 divided in the latter. In the Antiarcha, again, the interorbital or 

 pineal plate is always loose so far as known, while it is fixed in all 

 Osteostraci except Tremataspis ; but even when fixed the outline 

 can be readily distinguished in some examples of Cephalaspidse, and 

 there is a fine fragment of Eakeraspis displaying the contour of 

 this element in the Ludlow Museum. Ln the absence of narial 

 openings in the cranial shield, both types also agree ; and the 

 olfactory organ, if present, must thus have retained its embryonic 

 situation on the ventral aspect immediately in front of the mouth. 

 In short, so far as the shield can afford a clue to the essential soft 

 parts, these were arranged upon one and the same plan in Hetero- 

 traci, Osteostraci, and Antiarcha. 



Finally, the caudal region, as known in Cephalaspis, may be 

 compared with that of Pterichthys in all essential particulars ; and 



1 T. H. Huxley, Quart. Journ. Geol. Soc. vol. xiv. (1858), pp. 267-280, 

 pis. xiv., xv. 

 a E. E. Lankester, 'The Cephalaspidae ' (Mon. Palaeout. Soc. 1867-69). 



