418 



J. W. DAVIS ON THE PISH-EEMAINS OF 



Besides Hybodus minor, the only spine of this genus described 

 by Prof. Agassiz from the Aust bed was H. Iceviusculus*. The 

 original which served for the description of the latter was a small 

 fragment about half an inch in length, in the museum at Bristol. 

 It is described as having smooth sides, slightly compressed, with an 

 internal cavity rounder than the external form of the spine. There 

 are denticles along the posterior border ; and these are long, pointed, 

 and recurved towards the base. 



The spine I have from Aust, the only other small form of 

 Hybodus which I have seen, differs in every respect, except size, 

 from H. Iceviuseulus of Agassiz. Its sides are deeply furrowed ; the 

 posterior denticles are short and blunt ; and the internal cavity is 

 much longer than broad in section. 



The number of well-defined species of teeth of Hybodus found in 

 the Rhaetic beds of Aust would lead to the inference that there 

 should be a similar variety in the fin-defences of the fishes. Under the 

 most favourable circumstances it is an extremely difficult matter to 

 correlate the dermal defences, either spines or scutes, of the Sela- 

 chians with the teeth of the same genus ; but in this instance the 

 difficulties are greatly increased by the roiled and mixed state in 

 which the specimens are found ; and it appears improbable that 

 remains will be discovered whose relationship will be rendered 

 certain by the position or circumstances under which they are dis- 

 covered. The pittings along the grooves suggest the name punctatus 

 as appropriately designating this spine. 



Remarks on the Genus Nemacanthus, Ag. 



This genus was formed to embrace two species of fossil spines of 

 Selachians found in the bone-bed at Aust. The spines are about 5 

 or 6 inches in length and '7 inch in breadth in the larger species, viz. 

 JV. monilifer, and little more than half that size in the second one, 

 N. filifer. The genus is characterized by the spine having its sides 

 much compressed, and finely striated, with a small posterior cavity 

 reaching half the length of the spine ; where the cavity terminates on 

 the posterior surface there commences on the sides a number of 

 rounded tubercles : they originate near the anterior surface, extend 

 obliquely across the spine, and run in parallel lines thence to its 

 apex, a row extending along the junction of the lateral with the 

 posterior face having some resemblance to a row of small blunt 

 tubercles along each side. Along the anterior portion of the spine 

 there extends a round keel, which is marked off from the body of 

 the spine by a lateral canal along each side. In the larger species 

 the lateral keel is of about the same diameter as the tubercles arranged 

 along the sides. The smaller species, N. filifer, differs from the 

 larger, N. monilifer, in the tenuity of its anterior keel, the smallness 

 of the tubercles on its sides, and also of those extending along the 

 posterior edges. 



* Poissons Fossiles, tome iii. p. 46, tab. 10. figs. 24, 25, 26. 



