THE LIMBS OF FISHES. 39 



tliese follow some small aclclitional radlaUa, which remain car- 

 tilaginous, and are embraced by the bases of the fin-rays. In 

 the other Ganoids the propterygial basale disappears, and 

 some of the radialia, pushing themselves between the meso- 

 pterygial and metapterygial basalia, articulate directly with 

 the pectoral arch. The mcsopterygial basale is embraced by, 

 and becomes more or less incorporated with, the large ante 

 rior 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. Four bones, of very similar 

 general form, usually articulate with the pectoral arch, be- 

 neath and behind 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 lowermost 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.) f I ij, 



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, rep- 

 resenting a clavicle, with supra-clamcidar audi post-clavicular 

 ossifications, are added. 



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

 tion of the Teleosteans, the pelvic fins are situated far back 

 on the mider side 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 Vertebrate Exoskeleton. — The Exoskeleton never at- 

 tains, in vertebrated animals, the functional importance which 

 it so frequently possesses among the Invertehrata, and it va- 

 ries very greatly in the degree of its development. 



The integument consists of two la3^ers — 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 vascu- 

 lar tissue, the dermis^ composed of more or less completely- 



