226 



VERTEBRATE SKELETON 



forwards so that the skeleton appears symmetrical, a secondary 

 diphycercy. 



Some Elasmobranchs (Spinacids, Cestracionids, some Holocephals) have a 

 large spine in front of the dorsal fin (fig. 238). This is a modified dermal scale 

 supported on the anterior side of a pterygophore. The spine exists independent 



Fig. 238. — Skeletons of dorsal fins of (.4) Heplanchus, (B) Zygcena, (C) Acanthias 

 (Mivart, '79); (D) , Helerodonlus (Daniel, '15). b, basalia; r, radialia; s, spine. 



of a fin on the tail of sting rays. The Elasmobranch pterygophores arise from a 

 continuous procartilage, the separate rays appearing on chondrification. The 

 radials are often shortened to polygonal plates (fig. 238, D). 



Characteristic of most Ganoids are fulcra, two rows of overlapping modified 

 scales (those of the two sides usually united to an inverted V) on the anterior 



margin of the median fins. Acipenser 

 has the caudal fin supported on carti- 

 lage rods (either radials or separate 

 spinous processes). The dorsal fin of 

 Polypterus, continuous in the embryo, 

 is divided into a number of finlets (fig. 

 240), each with an anterior spine (mod- 

 ified scale) supported on an axinost 

 and bearing a series of lepidotrichia on 

 its posterior border. 



In Teleosts, where the rays are 

 mostly lepidotrichia, remnants of cera- 

 titrichia persist at least for a time, be- 

 tween the lepidotrichia of the two sides 

 and also in the adipose fin of Salmonids. 

 The sucker of remoras is a peculiar flattened fin in which the axinosts are trans- 

 verse bars with lepidotrichia between them. 



Fig. 239. — Two finlets of Polypterus 

 (Goodrich, '09). a, actinost; /, lepi- 

 dotrichia supporting fin-membrane; 5, 

 spine of finlet. 



