Mr. A. 8S. Woodward on the Myriacanthide. oie 
with a fine granulated ornament; underneath it is beset with 
relatively large pointed tubercles, and its tapering anterior 
extremity reaches almost as far forward as the end of the 
nasal prolongation. ‘The dorsal fin-spine, as described by 
Egerton, exhibits all the essential characters of Myrtacan- 
thus ; and so far as the imperfect type specimen of J/. granu- 
latus, Ag., is capable of comparison there appears to be a 
sufficiently close agreement to justify the assumption of 
specific identity. The left mandibular tooth is exposed from 
the inner aspect and seems to exhibit three distinctly sepa- 
rated narrow tritoral areas. The palatine teeth are too 
imperfect to reveal more than the fact that they are thin 
plates with one lateral margin deflected. 
A second and very imperfect specimen of ‘‘Ischyodus ortho- 
rhinus”’ in the Egerton Collection (no. P. 1158) exhibits a 
small dermal plate with granulated ornament; and a third 
fragmentary specimen in the Enniskillen Collection (no. 
P. 4575), proved by the frontal spine to pertain to the same 
species, makes known a few further details in regard to the 
dentition. 
The last-mentioned fossil exhibits from the anterior outer 
aspect the imperfect remains of the mandible, with its two. 
large dental plates in position; and, evidently somewhat dis- 
placed, there lies upon the oral margin of the right lower 
dental plate a small narrow tooth, at first sight suggestive of 
the incisor of a rodent mammal. ‘This tooth, however, is 
bilaterally symmetrical, and must have occupied a median 
position in the jaw; it consists mainly of ‘ cement,” though 
exhibiting a thin band of dentine upon the middle of its inner 
face, and the gently rounded upper end has obviously been in 
function. Dental plates that are certainly referable to the 
upper jaw are also seen; but only one small pair, which 
appears to be vomerine, displays any recognizable characters. 
Hach of these plates is broad in its posterior two thirds, with 
traces of tritoral areas; and the narrow anterior third, with 
~ parallel sides, is marked by a few large transverse ridges of 
dentine. 
We have already identified the dorsal fin-spine of ‘“Jschy- 
odus orthorhinus”’ with the Ichthyodorulite Myriacanthus 
granulatus, long ago made known by Agassiz; and it now 
remains to ascertain whether the dentition of the fish, as just 
described, is identical with, or closely similar to, any type of 
dentition already discovered. 
In this connexion the so-called Prognathodus* at once 
* Sir P. Egerton, ‘On Prognathodus Guenthert, Egerton, a new Genus 
of Fossil Fish from the Lias of Lyme Regis,’ Quart. Journ. Geol. Soc. 
vol. xxviii. (1872), pp. 233-256, pl. vill. 
