On the Structure and 121 



As the aiitorioi' one loses its dentielcs or becomes worn or broken, 

 it falls, and is succeeded by 'dwoihQv from hehind. Yet several 

 may be in existence and effective at the same time, all arising 

 from a common segmented bony base which grows by additions 

 to its posterior extremity* All this is true of the spines of 

 Edestns, if we are right in locating tliem in the position of the 

 second dorsal fin on the back or tail of a plagiostome fish. 



Hence until further light shall be thrown n})onthe interesting 

 question of the homologies and functions of Edestus, we may re- 

 gai'd them as the post-dorsal spines of lai-ge cartilaginous fishes, 

 of wliich the other parts are yet unknown, and may suppose that 

 they were used for attack and defense, like the spines of Trygoii 

 or Acanthuru!^. 



" On the tail of Heliohatis, Marsh, a freshwater ray from the Eocene 

 Green river beds of Wj^oming, I have seen three spines which must have 

 been in service at the same time. 



EDESTUS GIGANTEUS, sp. n. 

 Plate VI, Fig. 1. 



Spine veiT large, 18 inches or more in length, by 7^ inches in 

 bread til to top of denticles, and 2 inches in thickness at centre ; 

 foi-m strongly arched, section spatnlate in the middle, lenticular 

 at ba.-e ; lateral surfaces of bony portion vermicularly rough- 

 ened ; segments narrow, running far back, about f of an inch 

 wide, in the middle of the spine 10 inches from summit of den- 

 ticle to lower margin ; denticles 3-g- inches long by %\ inches wide 

 at base, triangular in outline, crown about as broad as high, 

 base prolonged backward and downward into a simple curved 

 point ; mai-gins set with 15 to 18 strong, rounded, sharp, com- 

 })ressed serrations. 



This remarkable spine differs from the other species of the 

 genus not only by its greater size, but by the form of its enam- 

 eled denticles. It approaches nearest to Edestus vorax, Leidy, 

 described in the Proceedings of the Academy of Natural 

 Sciences, Philadelphia, Vol. VII, p. 414, and in the Journal of 

 tlr' Academy. Second Series. Vol. Ill, p. 159, PI. XV, but is 

 distinguislied from that species by its larger size, more pro- 

 longed segments, and especially by the outline of the bases of the 



