EASTMAN : CARBONIFEROUS SHARKS. 73 



dorsal fin, and bury it in the soft parts, all except the denticles, the elongation 

 backward by the successive addition of sheaths and denticles becomes in- 

 telligible and natural." 



Correct in his determination of the anterior and posterior extremities 

 in E. heinrichi as such, Newberry yet deemed it "scarcely comprehen- 

 sible " that any form of symphysial teeth should have their roots so 

 enormously produced and enlarged in front ; and accordingly he rejected 

 this for the spine hypothesis of Leidy and Owen. The latter theory, 

 however, involved certain difficulties of its own, which the same author 

 thus comments upon : ^ 



" It is also apparent that the growth of this organ was by additions to the 

 summits of successive sheaths, each of which carried a denticle. This is strik- 

 ingly different from the mode of growth of all sharks' spines kno^Ti, as these 

 increase by additions to the base, and are thus pushed upwards and length- 

 ened. The same is true of all rostra which are used as weapons of defence or 

 offence. If we consider the segments of Edestus as the homologues of a dental 

 series we encounter the same difficulty. . . . We are therefore compelled to 

 conclude that the spine was buried in the integuments throughout its entire 

 length, the enamelled denticles alone projecting above the surface to form a saw, 

 which would be a terrible weapon if placed upon some flexible portion of the 

 body where it could be used with freedom and power. The extremity of the 

 spine may have lain in a sheath, from which it could be partially erected by 

 muscular action and used as the lancet of the surgeon fish (Acanthurus) is." 



It is rather curious that so close an observer as Newberry, and fol- 

 lowing him Miss Hitchcock, should have entertained the idea that some 

 of the segments of Edestus were freely movable upon one another, and 

 possibly erectile. To notice Miss Hitchcock's theory ^ briefly, in pass- 

 ing, it must be said that in so far as she conceived Edestus to be homol- 

 ogous with the intermandibular arch of Onychodus, — a Crossopterygian 

 ganoid, — her interpretation was at fault ; but at the same time her 

 reference of the series to the median line, in front of the lower jaw, was 

 a close approximation to the truth, as has been finally revealed through 

 a study of Campodus. And certainly without a knowledge of the latter, 

 comparisons of Edestus with the symphysial series of Cestracion, Car- 

 charias and other recent sharks, would have appeared fantastic in the 

 extreme. 



The way is now prepared for a more intimate comparison of the seg- 



1 Loc. cit. (1889), p. 223. 



2 Proc. Amer. Assoc. Adv. Sci.. 1887 (1888), p. 260. Amer. Nat., Vol. XX. 

 (1887), p. 847. 



