177 



The pisiform of the carpus is a sesamoid bone (^i.e., a mem- 

 brane bone developed in a tendon as a result of strain or pres- 

 sure), while the centrale often fuses with the carpale^ to form 

 the OS magnum. 



The median or unpaired fins which develop from the dorsal 

 pair of lateral folds and the postanal part of the ventral folds 

 occur only in the ichthyopsida. The 

 result of such a union of folds (p. 

 172) would be to produce a fin in the 

 median line which, beginning on the 

 back, should continue around the 

 tail and forward upon the ventral 

 surface as far as the vent. Such a 

 continuous fin occurs in the cyclo- 

 stomes, larval amphibia, and many 

 other forms ; but usually it is inter- 

 rupted, and thus divided into dorsal, 

 caudal, and anal (on the ventral sur- 

 face) fins. In the amphibia these 

 fins are without skeletal support ; 

 but in the fishes a regular skeleton 

 is formed, consisting of segmentally 

 arranged basalia and radialia like 

 those of the primitive paired fins, 

 and besides, a system of dermal fibrous supports. Occasionally, 

 however, there is to be found an intercalation of radialia, these 

 sometimes being at least twice as numerous as the somites. 



Fig. 191. Dorsal vertebrse of 

 Pleuracanthus, after Fritsch. c, 

 notochord; h, hasmal arch; n, 

 neural arch; r, radialia of dorsal 

 fin, showing intercalation of ele- 

 ments. 



