314 Dr. A. S. WooJvvaiJ — The Rdalions of 



the wliole of the succeeding Jurassic period was spent by the 

 liigliest fishes in improving and fiiiisliing their internal 

 sk-eleton, while their external bony armour began almost 

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

 Cretaceous ])eriod the most advanced members of the class 

 had already become true bony fishes or Tel-osteans. Having 

 attained that stage o£ complexity, 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. Simul- 

 taneously many of the fin-rays were modified into spines, and 

 there was a constant tendency for the external bones ami 

 scales to become spinose. At all stages of tliis 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 armour so long as they remained compara- 

 tively 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, alter 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 armour less useful ; that, finally, in the 

 highest types the vertebra? and some of the fin-rays were 

 reduced to a fixed and practically invariable number for each 

 family or genus, while there was a remarkable development 

 of spines. As survivors of most of these stages still exist, 

 the changes in the soft parts which accompanivid the succes- 

 sive advances in the skeleton can be inferred. Hence Paheon- 

 tology furnishes a sure basis for a natural classification m 

 complete accord with the develo[)ment of the group. 



