AC AXTHODID2E. 
3 
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-hone. 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 b 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,” Chlamydoselciche. 
The cast of a pair of large oval lobes has been observed in the 
head of a Siberian species 1 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 
hsemal 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 ( b ), 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 “styliform bones forming the rami of the 
lower jaw ” of Egerton. 
2 J. V. Rohon, Mem. Acad. Imp. Sci. St. Petersbourg, [7] vol. yxxYi. no. 13 - 
(1889), p. 4, pi. i. figs. 8, 9. 
