54 



Geological Society. 



after pointing out the discrepancies both of form and structure which 

 they present, suggests the propriety of withdrawing them from the 

 genus Chimcera, under which they have hitherto been arranged. 

 The remainder of the memoir is devoted to a comparison of the fossil 

 species with each other, and the author concludes by proposing to 

 class them under three genera, as shown in the following tabular 

 arrangement. 



(1.) Ischyodus {la\vs robur, olovs dens). 



Two intermaxillary and two maxillary plates in the upper jaw ; 

 two maxillary plates in the lower jaw ; intermaxillaries thick and 

 strong, truncated more or less obliquely at their extremities. Struc- 

 ture : horizontal laminae inclosed by parietes of coarse fibrous dentine. 



Upper maxillaries : triangular plates articulating with each'other 

 and the intermaxillaries on the medial line of the palate ; upper sur- 

 face provided with a deep sulcus parallel to the symphysis for attach- 

 ment to the jaw ; under surface with four triturating prominences, 

 one in advance, one on the outer margin, and two side by side near 

 the base, the larger one occupying the inner position ; structure of 

 the tubercles coarse and tubular ; the remainder of the teeth fibrous 

 and bony. Lower maxillaries : large and broad, formed for crushing 

 rather than cutting ; two tubercles, one at the heel, the other in ad- 

 vance ; symphysis broad ; the base invested by the membrane of the 

 mouth, the crown by a coat of hard enamelloid dental substance ; 

 structure of the anterior angle as in the intermaxillaries, of the re- 

 mainder as in the upper maxillaries ; position of the plates more or 

 less oblique. 



SPECIES. 



Ischyodus, Egerton. 

 Agassizi, Buckl. . . . 

 Beaumonti, Egert. . 

 brevirostris, Agass., 

 Bucklandi, Egert. . 



Colii, Buckl 



curvidens, Egert. . 

 Dutetrii, Egert. . . , 

 Duvernoyi, Egert. , 

 Egertoni, Buckl.. . . 

 emarginatus, Egert. 

 falcatus, Egert. . . . 

 helveticus, Egert. . 

 Mantelli, Buckl. . . . 

 neglectvs, Egert.. . . 



Oweni, Buckl 



psittacinus, Egert. . 

 rugulosus, Egert. . 

 Tessoni, Buckl. . . . 

 Townshendi, Buckl. 

 Sedgwicki, Agass. . 



Chalk- marl .... 

 Kimmeridge clay 



Gault 



Great oolite. . . . 



ditto 



ditto 



Kimmeridge clay 



ditto 



ditto 



Great oolite. . . . 



ditto 



Molasse 



Chalk 



Great oolite .... 



ditto 



ditto 



ditto , 



Oolite 



Portland , 



Greensand 



Hamsey. 

 Boulogne. 

 Folkstone. 

 Stonesfield. 



Ibid. 



Ibid. 

 Boulogne. 



Ibid. 

 Shotover. 

 Stonesfield. 



Ibid. 

 Argovie. 

 Lewes. 

 Stonesfield, 



Ibid. 



Ibid. 



Ibid. 

 Caen. 

 Milton. 

 Cambridge. 



