INTRODUCTION. 
XY11 
7 
Sauropsis, apparently ranging from the Upper Lias to the Lower 
Kimmeridgian, and Euthynotus, confined to the Upper Lias, are the 
least specialized genera of Pachycormidse. Pachycormus itself is 
known only from the Upper Lias, and is a little more modified, 
having lost the pelvic fins. Hypsocorrrms, from the Oxfordian and 
Kimmeridgian, exhibits more powerful teeth than any of the earlier 
genera, these fixed to the base. of sockets which are incomplete on 
the inner side ; and there is one species, H. leedsi, possessing a 
slightly more prominent rostrum than any earlier fish. The latest 
genus Protosphyrcenci, known only by comparatively fragmentary 
specimens from the Cretaceous both of Europe and America, 
nearly always exhibits a much more produced rostrum, while its 
large teeth are fixed in complete sockets; and the pectoral fins of 
this fish are still narrower and more elongated than those of its 
forerunners in the Jurassic period. 
If the interpretation of the Pachycormidae suggested in this 
Catalogue prove correct, there is thus most distinct evidence of 
gradual progression as they are traced upwards in their range. 
Aetheospondyli. 
The recognition of this group is a confession of ignorance. As to 
the origin and immediate relationships of the Aspidorhynchidae 
and Lepidosteidae, there is still not the faintest clue. That these 
two families are close allies is generally admitted; that they are 
connected in some way with the Protospondyli as above defined, is 
also a common belief. They are peculiar, however, in exhibiting 
annular or solid centra throughout the length of the vertebral 
column, no alternating pleurocentra and hypocentra even in the 
caudal region; and they cannot be referred to the Isospondyli on 
account of the complexity of the mandible. They may thus be pro¬ 
visionally assigned to an intermediate, though scarcely annectant 
group, which is named Aetheospondyli in allusion to the unique 
variations of the vertebral centra from the biconcave or amphi- 
coelous to an opisthocoelous type. 
Asjoidorhynchidce. 
The Aspidorhynchidae are long-bodied fishes with deep ganoid 
flank-scales, small fins, and elongated snout, ranging from the 
Bathonian to the Upper Cretaceous inclusive. They are remarkable 
for the possession of a median presymphysial bone in the mandible. 
part in. b 
