ANATOMY C»F VERTEBRATES. 160 



fest, like other newly introduced organs, the principle of vegetative 

 repetition, there being three or four horny filaments to each carti- 

 laginous ungual phalanx. 



On the fore part of the coracoid arch, near to the prominence 

 supporting the fin, there are developed a vertical series of small 

 bony cylindrical nuclei in the substance of the cartilage in most 

 Sharks. In the Rays the coraco-scapular arch forms an entire 

 circle or girdle attached to the dorsal spines : it consists of one 

 continuous cartilage in the Rhinobates, but in other Rays is divided 

 into coracoid, scapular, and suprascapular portions, the latter 

 united together by ligament. The scapula and coracoid expand 

 at their outer ends, whore they join each other by three points, to 

 each of which a cartilage is articulated homologous to the tln-ee 

 alcove described in the Shark, and which immediately sustain the 

 fin-rays. The posterior cartilage answering to the upper one in 

 the Shark curves backward and reaches the ventral fin : the an- 

 terior cartilage curves forward, and its extremity is joined by the 

 antorbital process as it proceeds to be attached to the end of the 

 rostral cartilage ; the middle proximal cartilage is comparatively 

 short and crescentic, and sustains about a sixth part of the fin-rays, 

 which are the longest, the rest being supported by the anterior 

 and posterior carpals, and gradually diminishing in length as they 

 approach the ends of those cartilages. 



Developement by irrelative repetition of parts reaches a maximum 

 in the present plagiostomous group. In the common Ray, fig. 64, 

 there are upwards of a hundred many-jointed fingers in each pectoral 

 limb : but all are bound up in a common function of the simplest 

 kind. 



§ 40. Pectoral limb of Reptiles. — The other route of develope- 

 ment from the prototypal condition exemplified in fig. 101, A and 

 C leads to a differentiation of the several divisions and parts of the 

 limb, and their adaptation to particular functions or parts of com- 

 Ijined and varied mechanical actions. 



The first step, as manifested in the Amphiume, ib. B, c, is the 

 formation of a long inflexible segment, as a lever of greater resist- 

 ance, 53 and 65 ; tliis is followed l)y a pair of similar, but shorter 

 cylindrical bones, each sustaining a ray of few joints. The 

 proximal bone assumes through ulterior developements the special 

 name ' humerus,' or arm-bone, with the symljol 53, in the fore 

 limb; and of 'femur,' or thigh-bone, with the symbol 65, in the hind 

 limb. The two bones of the next segment become, in the fore 

 limb 'radius,' 54, and ' ulna,' 55 — collectively, antibrachium or 

 ' fore-arm ; ' in the hind limb, tibia, 66, fibula, 67 — collectively. 



