20 



PISCES. 



To begin, then, with the median fins, reference may be 

 made first to the earliest clearly decipherable example of an 

 Elasmobranch median fin shown in the accompanying illustra- 

 tion (fig. 17). It belongs to the genus Pleur acanthus, of 



FIG. 17. 



Pleuracanthus (Xenacanthus) decheni ; portion of caudal region, about nat. size. 

 L. Permian ; Bohemia, ch., position of the notochord, bounded by 

 neural and haemal arches ; /, anal fin ; p, r, dorsal and ventral portions 

 of caudal fin. (After A. Fritsch.) 



Carboniferous and Permian age. To refer to the dorsal, it will 

 be observed that the fin-fold itself is stiffened by numerous 

 filiform dermal rays, and is supported by a series of parallel 

 vertical rods of cartilage, each segmented into three pieces. 

 The outer short piece is within the fin-membrane ; the other 

 pieces are within the body between the muscles, and the outer 

 (distal) element is termed a baseost, the inner (proximal) 

 element an axonost. In the caudal region these supports are 

 equal in number to the neural spines and directly apposed to 

 the latter; while in the abdominal region they are exactly 

 twice as numerous, and instances are supposed to occur in 

 which each neural spine is bifurcated to meet the extremities 

 of the two fin-supports corresponding with it. The same direct 



