THE ENDOSKELETON 



175 



distinct bone save in the monotremes. In others it ossifies 

 from a separate center, but soon fuses with the scapula to form 

 the coracoid process. The clavicle is large and well developed 

 in those forms in which strength of shoulder is especially re- 

 quired, as in most cases among the Rodentia, Insectivora and 



FIG. 46. Anterior fins of fishes. 



(A) Dog-fish (selachian). (B) Ceratodus (dipnoan). 



s, scapula; ss, suprajcapula; c, coracoid; p, propterygium ; ms, mesopterygium; 

 mt, raetapterygium; rod, radials. In B there are radials on both sides of a central 

 axis. 



Primates, but is rudimentary in Carnivora, and is entirely 

 wanting in hoofed mammals and in Cetacea. It is stated by 

 some investigators that the mammalian clavicle is in develop- 

 ment a compound piece, formed, as in amphibians, of a carti- 

 laginous core, overlaid by a dermal element. If this be so, the 

 former may be the procoracoid, and the latter the true clavicle, 

 but there is some doubt concerning the actual conditions of 

 development, and the whole matter needs further investigation. 

 The early history of the free limbs has not been wholly 

 deciphered. The fins of fishes exhibit a great variety of form, 

 based upon a series of fin-rays, either distinct or united to 

 basal pieces. Of these latter the posterior limb of selachians 

 shows one, the basi-pterygium f and the anterior limb three, 



