INTRODUCTION. 
SIX 
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 1 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 Asterolepidae) 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 , 
Diclymaspis , and Trematcispis 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 dorsallv and ventrallv by an armature that meets in 
a close suture laterally. As clearly shown in Trematcispis , 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 plates, 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 Trematcispis; but even when fixed the outline 
can be readily distinguished in some examples of Cephalaspidse, and 
there is a fine fragment of Eukercispis displaying the contour of 
this element in the Ludlow Huseum. In 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. Cteol. Soc. vol. xiv. (1858), pp. 2(57-280, 
pis. xiv., xv. 
2 E. E. Lankester. ‘The Cephalaspidse’ (Mon. Palaeont. Soc. 1867-69). 
