THE ENDOSKELETON 



171 



although here the fault lies in the derivation of ganoid condi- 

 tions directly from the primitive form without accounting 

 for that of the selachians. The first of these figures is that 

 of Acipenser, the sturgeon (Fig. 44, a), and still represents 

 the primitive condition of parallel fin-rays, save that several 

 of the anterior ones have fused in order to meet the greater 

 strain imposed upon them. This tendency has progressed still 

 farther in a related ganoid, Scaphyrhynchus (Fig. 44, b), 

 where the proximal portions of nearly all the rays are included 

 in the heavy basal piece, which bears both the distal portions 

 of the rays of which it is composed as well as the few original 

 rays which have not entered into its formation. In this an 

 important step is the formation of a pair of little pieces, which 

 are segmented off from the proximal ends of the two basal 

 pieces, and which serve to interpret the condition found in 

 Polyptcrus, in many ways the highest of the living ganoids 

 and the one nearest the amphibians (Fig. 44, c). On this the 

 basal piece of each fin has partly ossified and becomes a long 

 limb-bone highly suggestive of a femur, and the two are at- 

 tached to a small mid-ventral piece, a rudimentary girdle, 

 which is divided by a suture into two portions, plainly the same 

 as the two small inner pieces of Scaphyrhynchus, here united 

 to form a rhomboid plate. 



The derivation from this of the condition found in the 

 urodele Necturus becomes at once evident in a comparison 

 of the two (Fig. 44, c and d), in the latter of which the 

 steps in advance consist of the enlargement of the plate, its 

 connection with the vertebral column through two small 

 processes, the ilia, that extend dorsally, and the appearance 

 of two centers of ossification posteriorly. In this the real 

 transition from the fish type to that characteristic of the higher 

 vertebrates consists of the direct connection between the hip- 

 girdle and the vertebral column, a condition never found in 

 fishes. That this attachment has been newly acquired at the 

 stage represented by Necturus is evidenced by the lack of 

 difference between the sacral vertebra, to which the attachment 

 is made, and the adjacent ones, as well as by the frequency of 



