278 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 5, 



surface, on the other hand, being rather convex than concaYe. 

 A portion of the membranous fin is seen attached to the hinder 

 edge of the spine, and is composed of fine-grained shagreen ; im- 

 pressions are faintly seen in other parts of the specimen, which 

 show that the dermal covering in general was of the same cha- 

 racter. The outline of the mandibular region is fortunately pre- 

 served, and exhibits a very singular truncated form of lower lip. 

 The labial and mental angles are right angles, and connected by 

 a straight line one inch in length ; the upper and lower margins 

 are nearly parallel for a distance of one inch from the mentum ; 

 so that the area of the anterior mandibular projection was a 

 square of one inch. The dental apparatus is rather dislocated ; but 

 the component parts remaining are well preserved. There is an 

 indistinct impression of the left maxillary plate in its proper posi- 

 tion ; but its feUow of the right side (1 e) is thrown back below the orbit. 

 This measures one inch and a quarter in length by three lines in 

 width, being of slighter proportions than the corresponding denticle 

 of Ischyodus Egertoni, which it most resembles. The premaxillaries 

 are absent. The mandibular plates (1/) are reversed, but retain their 

 relative positions. Each measures one inch and a half in length, 

 and is one inch deep at the symphysis. The dental apparatus cor- 

 responds in all respects with the characters assigned by me to 

 Ischyodus, to which genus I refer this specimen ; but as it differs 

 specifically from those already described, I have selected a name signi- 

 fying its peculiar deviation from the recent types in the size and 

 form of the rostral appendage. 



This remarkable specimen has been secured for the fine collec- 

 tion of fossil ichthyology in the British Museum. 



DESCRIPTION OF PLATE XIII. 



Fig. 1. Ischyodus orikorhinus, reduced one-third: a, labial prolongation; 

 b, frontal appendages ; c, dorsal spine ; d, articulation of the dorsal 

 spine ; e, maxillary plate ; /, mandibular plates. 



Fig. 2. Rostral tubercle, magnified. 



Fig. 3. Outline of the anterior parts of Callorhynchus antarcticus, reduced one- 

 half: a, labial prolongation; h, frontal appendage; c, dorsal spine; 

 d, articulation of dorsal spine. 



Discussion. 



Dr. GvNTHER commented on the interest of this discovery, as in 

 no other Sharks is the same articulation of the dorsal spine as that 

 described in the paper to be found. He inquired whether the 

 granulated plate supposed to be dorsal might not be a part of the 

 armature of the lateral line, as in Sturgeons. He thought that the 

 Chimseroids would eventually prove to be intermediate between the 

 Ganoid and Shark types, and that all belonged to one subclass. 



Mr. GwYN Jefpeexs inquired what other remains were found with 

 these fishes such as might represent the food, molluscan or otherwise, 

 on which they lived. 



