THE ENDOSKELETON 



173 



it is probable that its early history is similar, and that it was 

 formed by a coalescence of basal portions of the fin-rays, as in 

 the other case, its size and shape being modified in accordance 

 with differences in function. In the adult selachians it ex- 

 tends dorsal to the insertion of the fin into long points, and 

 the two halves become distinct, forming a symphysis at the 

 mid-ventral line, thus somewhat resembling a pair of man- 

 dibles. Each lateral piece is termed scapulo-coracoid, as a 

 suggestion of the two elements to which it is to give rise, 



FIG. 45. Shoulder-girdle of fishes. 



(a) Dog-fish (selachian). (b) Polypterus (ganoid). (c) Cod (teleost). (d) 

 Ceratodus (dipnoan). 



s, scapula; ss, suprascapula ; c, coracoid; cv, clavicle; ct, cleithrum; xx, supra- 

 cleithra. 



but, as in the case of the primordial skull, there is no sug- 

 gestion of boundaries between the two portions, although it 

 may be vaguely indicated by the point of insertion of the fin, 

 the portion dorsal to it being the scapular portion, and the 

 other the coracoid. 



In ganoids there becomes associated with this cartilaginous 

 girdle a series of dermal bones, formed, as elsewhere, by the 

 fusion of scales. There are two of these bones investing each 

 lateral piece, the clavicles being ventral and the cleithra lat- 

 eral to these. Of these the former are a little larger, and 

 form the symphysis in the mid-ventral line, excluding the 



