﻿ACANTlIODll)^. O 



minute granular calcifications. There is no definite evidence of 

 membrane-bones bordering the mouth ; but in genera which possess 

 teeth (e.g. Acanthodopsis and Ischnacanthus) the oral margin both 

 of the upper and lower jaws is ensheathed in a well-developed mem- 

 brane-bone. In the small species from the Old Red Sandstone the 

 roof of the skull is distinctly covered with an irregular mosaic of 

 small dermal scales ; and in all the species a circumorbital ring of 

 four dermal plates is conspicuous. Between the rami of the lower 

 jaw, there occurs a pair of slender cartilages, not expanded at the 

 extremities, but firmly calcified ; and these are accompanied by a 

 sparse series of delicate rays in such a manner as to suggest that 

 they represent the ceratohyals l . The branchial arches, of which 

 there are five, are also calcified ; and on the hinder or convex margin 

 of each is arranged a close series of lanceolate appendages, having 

 the free extremity broader than the attached end, and not impro- 

 bably destined for the support of dermal flaps, resembling those 

 upon the gill-arches of the recent " frilled shark," Chlamydoselache. 



The cast of a pair of large oval lobes has been observed in the 

 head of a Siberian species 2 , these not improbably indicating the 

 form and proportions of part of the cerebral cavity. 



In the axial skeleton of the trunk the notochord is persistent, 

 and the arches are so rarely observable that they must have been 

 very slightly calcified. There are no traces of ribs, but a series of 

 slender neural arches is feebly indicated in a specimen from the 

 Calciferous Sandstones of Eskdale (no. P. 5979, p. 10) ; and stout 

 haemal arches are sometimes preserved in the region of the caudal 

 fin in examples of the type species from the Permian nodules of 

 Rhenish Prussia. 



Each of the fins, except the caudal, is provided with an anterior 

 spine, which resembles that met with in the dorsal fins of many 

 well-known Selachians, and is to be similarly regarded as an enor- 

 mous dermal ray. The fin-membrane is always stiffened by quadrate 

 dermal granules of the same nature as those covering the trunk, 

 and these are often arranged in regular lines simulating rays ; but 

 the pectoral and caudal are the only fins in which any traces of the 

 endoskeletal elements have hitherto been observed. 



At the base of each pectoral fin-spine (fig. 2) there abuts against 

 its posterior or concave border the broader end of a supporting 

 cartilage (6), which is elongated in a direction at right angles to the 

 spine (s), is constricted shortly above this articulation, and ends 



1 " Oral tentacles " of Huxley, and " styliforni bones forming the rami of the 

 lower jaw " of Egerton. 



2 J. V. Rohon, Mem. Acad. Imp. Sci. St. Petersbourg, [7] vol. sxxvi. no. 13 

 (1889), p. 4, pi. i. figs. 8, 9. 



b2 



