40 THE ANATOMY OF VEETEBRATED ANIMALS. 



basale is embraced by, and becomes more or less incorpo- 

 rated with, the large anterior fin-ray. 



From these Ganoids the passage is easy to the Teleostei, 

 in which, also, the mesopterygial basale always becomes 

 fused with the anterior fin-ray, whence the latter seems to 

 articulate directly with the shoulder- girdle. Pour bones, of 

 very similar general form, usually articulate with the pec- 

 toral arch, beneath and behiud the mesopterygial basale 

 and its fin-ray. At their distal ends small cartilaginous 

 nodules may lie, and these are embraced by the fin-rays. 

 Of these four bones, or partially ossified cartilages, the lower- 

 most and hindermost answers to the metapterygial basale 

 of the Shark; the others seem to be radialia. (See the 

 figure of the Pike's pectoral fin, infra.) 



The ventral fins have basal and radial cartilages and fin- 

 rays, more or less resembling those of the fore limbs. 



In most Ganoids and Teleosteans the pectoral and pelvic 

 arches are, in part, or completely, ossified ; the former fre- 

 quently presenting distinct scapular and coracoid bones. 

 To these, in all Ganoids and Teleosteans, membrane bones, 

 representing a clavicle, with swpra-clavicular and post-clavi- 

 cular ossifications, are added. 



In all Elasmobranchs and Ganoids, and in a large pro- 

 portion of the Teleosteans, the pelvic fins are situated far 

 back on the iinderside of the body, and are said to be 

 " ventral " in position ; but, in other Teleosteans, the ventral 

 fins may move forward, so as to be placed immediately 

 behind, or even in front of, the pectoral fins. In the former 

 case they are said to be " thoracic," in the latter "jugular." 



The Vertehrate Exoskeleton. — The Exoskeleton never attains, 

 in vertebrated animals, the functional importance which it 

 so frequently possesses among the Invertebrata, and it varies 

 very greatly in the degree of its development. 



The integument consists of two layers — a superficial, non- 

 vascular substance, the epidermis, composed of cells, which 

 are constantly growing and multiplying in the deeper, 

 and being thrown off in the superficial, layers ; and a deep 

 vascular tissue, the dermis, composed of more or less 



