PISCB6 127 



They inhabit the comparatively deep seas, ranging from 200 to 1,200 

 fathoms, though one species, Chimcsra colliei, lives at or near the 

 surface. In all of them the median fin system has two peculiarities; 

 a strong spine anterior to the front dorsal fin and long whip-like tail 

 with either a diphycercal or a slightly developed heterocercal fin. The 

 Holocephali may be dismissed then as an interesting but not especially 

 significant side-line of elasmobranch evolution. 



SUB-CLASS II. TELEOSTOMI 



This great group of fishes to which belong nearly all of the modem 

 species except the Elasmobranchii, consists of a few fragmentary 

 vestiges of formerly abundant fish faunas and a vast assemblage of 

 modem bony fishes. The lowest of the vestigial orders, the Crossop- 

 terygii and the Chondrostei, show many evidences of relationship 

 with the primitive sharks and very probably descended from some 

 primitive shark type at about the same time that the sharks gave off 

 the Batoidei and the Holocephali. It may be said in introducing the 

 Teleostomi that the most primitive order, Crossopterygii, is from the 

 standpoint of vertebrate evolution of very especial importance; for 

 the group seems to belong to one of the main-trunk-lines of evolution 

 and to be an important junction point for several branch-lines of 

 vertebrate phylogeny. It is now believed that the Crossopterygii gave 

 rise not only to the other orders of Teleostomi, including the modern 

 teleosts, but to the Dipneusti and to the first Amphibia. 



The principal morphological characters of the Teleostomi as a whole are: 



1. The process of ossification of a primitively cartilaginous endo- 

 skeleton has resulted in the appearance of a number of separate bones 

 in the skull, and the jaws have also ossified more or less completely. 

 The roof of the chondrocranium has been covered over by a set of 

 dermal investing bones without teeth or denticles. 



2. The primary jaws (both upper and lower) have been covered 

 over and reinforced by tooth-bearing investing bones, developed in the 

 dermis. 



3. The operculum over the gills is supported by a more or less 

 elaborate dermal skeleton that becomes rather intimately associated 

 with the skull and lends a false appearance of complexity to the latter. 



4. The pectoral girdle, which is often attached to the skull, is 

 also made more complex by the addition of investment bones from 

 the dermis. The pelvic girdle is usually absent or vestigial. 



