18 PISCES. 



the high degree of specialization observable in certain groups 

 of the existing fauna. 



Again, by the nature of the exoskeleton two groups both of 

 the autostylic, and of the hyostylic fishes, can also be dis- 

 tinguished. Some families exhibit mere "placoid" dermal 

 calcifications, traversed by delicate branching nutritive canals, 

 these isolated plates never uniting even in the region of the 

 branchial apparatus to form any covering of the clefts ; other 

 families are well encased in dermal and membrane bones, which 

 have a definite symmetrical arrangement, and consist, at least 

 in their basal layer, of tissue with distinct bone-cells, which 

 are often disposed in haversian systems. Among autostylic 

 fishes, the Chimseroids are thus readily separated from the 

 Dipnoans ; while among hyostylic fishes, the Elasmobranchs are 

 sharply defined from the Teleostomes (to use Owen's term for 

 " Ganoidei" + " Teleostei "). 



The Class Pisces may thus be divided into the four sub- 

 classes of Elasmobranchii, Holocephali, Dipnoi, and Teleostomi; 

 and this is the only arrangement hitherto proposed which is in 

 full accord with the known facts of Paleontology. If the 

 common ancestors of all the four sub-classes could be found, 

 they would of course fall under the Elasmobranchii as here 

 defined ; and it may be that some of the early Palaeozoic 

 sharks already known bear an ancestral relation to more than 

 the sub-class to which they technically belong. Moreover, 

 there still remains some doubt as to how far certain Devonian 

 Crossopterygii (Holoptychiidae) constitute a link between the 

 primitive Teleostomi and Dipnoi ; while there are a few Holo- 

 cephali of Jurassic age which make at least a slight approach 

 to the Elasmobranchii in the nature of their dentition. Speak- 

 ing generally, however, it may be said that since the beginning 

 of Devonian times the four sub-classes in question have followed 

 distinct courses of development. The very earliest known 

 assemblage of undoubted fishes, presents well-characterized 

 examples of each of the four types (assuming that Chima?roids 

 can be determined from the evidence of detached teeth) ; and, 

 if the evolution of the paired fins is correctly represented 

 by the hypothesis that at present seems most plausible, the 



