INTRODUCTION. 
XIX 
the corresponding feature in Lepidosteus differs from that in all 
other surviving ganoids (cf. Proc. Zool. Soc. 1893, p. 565). 
ISOSPONDYLI. 
It is interesting to note that all the higher fishes, like the highest 
of the terrestrial Yertebrata, are characterized by a comparatively 
simple mandible. Only doubtful or extremely rare exceptions are 
recorded 1 to the rule, that each mandibular ramus in these types 
consists of two or three elements, an articulo-angular (or articular 
and angular) behind, a dentary in front. The circumstance is all 
the more remarkable because, on acquiring this simplification of the 
jaw, the Teleostomes seem to be infused with new vigour : vertebral 
centra invariably occur, at first as simple rings, then as robust 
amphiccelous bodies ; and a still more varied series of families arises, 
including analogues of all the principal modifications observed among 
the lower races, these being superinduced upon the new and 
advanced type of skeletal frame. 
The first and least specialized suborder of these higher fishes is 
that of the Isospondyli, thus named by Prof. E. D. Cope in allusion 
to the fact that the vertebrae are simple, not fused into a group 
behind the head or related in any way to arrangements for an 
osseous connection between the air-bladder and the organs of 
hearing. It comprises a large number of families, mostly Tertiary 
and Recent, and only the few Jurassic representatives of the sub¬ 
order with some of those of Cretaceous age are treated in the 
present Part of the Catalogue. 
Pholidophoridce. 
The earliest clearly-proved instance of the simple mandible just 
referred to, is that afforded by the small fishes of the genus Lepto- 
lepis occurring in the European Upper Lias. As already observed 
by Agassiz, however, the genus PholidopJiorus exhibits a very close 
resemblance to Leptolepis in general aspect, the osteology of the 
head being remarkably similar, vertebral rings being tolerably well 
ossified, the fin-fulcra very small and usually lost, while the scales 
are often extremely thin and deeply overlapping though for the 
most part united by a peg-and-socket articulation; and it is note¬ 
worthy that no indications of splenial and coronoid elements have 
1 E. s Arapairm, according to Owen, * Anatomy of Vertebrates,’ vol. i. 
p. 123, fig. 88 (1866). 
