560 PALEONTOLOGY 



period was spent by the highest fishes in improving and finishing 

 their internal skeleton, while their external bony armor began almost 

 universally to degenerate. Thus, by the early part of the Cretaceous 

 period the most advanced members of the class had already become 

 true bony fishes or Tcleosteans. Having attained that stage of com- 

 plexity they admitted of much more variation than formerly, and 

 then arose the immense host of fishes which characterize the Tertiary 

 period and the present day. For the first time in fish-history, there 

 were fundamental changes in the head. First, in some genera the 

 maxilla began to slip behind and above the premaxilla, so that it was 

 excluded from the gape. Next, in these and most other fishes, the 

 ear-capsules began to enlarge to such an extent that the original roof 

 of the brain-case eventually formed only an insignificant part of the 

 top of the skull. At the same time the lateral muscles of the trunk 

 extended forward over the cranial roof, and various crests arose 

 between them. Finally, it was quite common for the pelvic fins to be 

 displaced forward beneath the pectoral fins, while the vertebrae, as 

 well as some of the fin-rays, were usually reduced to a definite and 

 fixed number for each family or genus. Simultaneously, many of 

 the fin-rays were modified into spines, and there was a constant tend- 

 ency for the external bones and scales to become spinose. At all 

 stages of this progress there were, of course, stragglers left by the way; 

 and the modern fish-fauna is, therefore, a mixture of slightly modified 

 survivors of many periods in the earth's history. 



To state this brief summary in more general terms, fossils prove 

 that the earliest known fish-like organisms strengthened their external 

 armor so long as they remained comparatively sedentary; that next 

 the most progressive members of the class began to acquire better 

 powers of locomotion, and concentrated all their growth-energy on 

 the elaboration of fins; that, after the perfection of these organs, the 

 internal bony skeleton was completed at the sacrifice of outer plates, 

 because rapid movement necessitated a flexible body and rendered 

 external armor less useful; that finally, in the highest types, the ver- 

 tebrse and some of the fin-rays were reduced to a fixed and practically 

 invariable number for each family or genus, while there was a remark- 

 able development of spines. As survivors of most of these stages still 

 exist, the changes in the soft parts which accompanied the successive 

 advances in the skeleton can be inferred. Hence, paleontology fur- 

 nishes a sure basis for a natural classification in complete accord with 

 the development of the group. 



Now, fishes are aquatic animals and nearly all the fossiliferous 

 rocks were deposited in water. The past history of this chain of life 

 ought therefore to be almost completely revealed by the geological 

 records. Making due allowance for the imperfection of collections and 

 the accidental nature of the discovery of fossils, the general outlines 



