Xviii INTRODUCTION. 
Aspidorliynchus , with prominent rostrum, exhibits only one series 
of small teeth on the splenial bone, and has ring-vertebrae. It 
seems to be confined to the Jurassic; but a second genus, Belono- 
stomns, ranges from the Upper Jurassic to the Upper Cretaceous 
inclusive, and in the course of this range the delicate ring-vertebrae 
gradually pass into centra which are pierced only by a very delicate 
thread of notochord. It is also noteworthy that the enlarged 
splenial, covered with tubercular teeth, excludes the dentary from 
the margin of the mandible in the English Chalk species, Belono- 
stomus cinctus. Otherwise there is no special evidence of evolution 
in the family. 
It is just possible that the small Triassic fish Pholidopleurus , 
doubtfully assigned to the Pholidophoridae in the Catalogue (p. 479), 
may be a forerunner of the Aspidorhynchidae; but its cranial 
osteology is as yet too imperfectly known for comparison. 
Lepiclosieidce. 
Nearly all the known fossil remains of Lepidosteidae are mere 
fragments, none dating back further than the Lower Eocene. The 
only interesting fact to be deduced from these remains is, that fishes 
with scales and vertebrae identical with those of the existing 
American Lepidosteus inhabited Western Europe during the period 
of deposition of the Eocene and Lower Miocene deposits. Their 
range in the Old World thus corresponds with that of the fossil 
Amice already described. It is, of course, likely that their Mesozoic 
ancestors would not be recognized at first sight on account of 
the normal character of their vertebral centra; but the markedly 
forward inclination of the mandibular suspensorium seems to 
exclude comparison with all large-mouthed, conical-toothed 
Mesozoic ganoids except the Aspidorhynchidae, and these cannot be 
their direct progenitors for many obvious reasons. The irregular 
subdivision of the facial bones has been shown to occur commonly 
among highly-specialized ganoids, though Lepidosteus still remains 
unique in having this segmentation extended to the maxilla. As to 
the well-known remarkable features in the soft parts of the existing 
fish, it is unfortunate that Palaeontology cannot afford any definite 
information. Only one point may be mentioned, namely, that so 
far as can be judged from the cranium, the course of the olfactory 
nerves in Dcipedius differs from the course of these nerves in the 
closely-allied genus Lepidotus, exactly in the same way that 
