Mr. A. S. Woodward 07i the Mjriacantliida?. 277 



Avitli 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 Myriacan- 

 thus ; and so far as the imperfect type specimen of M. granu- 

 lafus, 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 ^^IscJiyodus 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 riglit 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. 

 Each 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 "/sc%- 

 odus orthorkinus " with the Ichthyodorulite Myriacanthus 

 (jranulatus, 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. Egertou, " Ou Proynathodus Gueniheri, Egerton, a new Genus 

 of Fossil Fish from the Lias of Lyme Regis," Quart. Journ. Geol. Soc. 

 vol. xxviii. (1872), pp. 233-236, pi. viii. 



