135 



edge 3 inches 6 lines ; anterior division of ditto 1 inch 8'Hnes ; width 

 of ditto 8 lines ; posterior division of ditto 1 inch 8 lines. This tooth 

 is broad and strong ; the exterior is marked with indistinct undu- 

 lations ; the depression at the heel is nearly horizontal. Inner sur- 

 face : symphj'sis rather oblique ; the triturating tubercles broad, and 

 worn down less obliquely than in C. townsendi, to which species this 

 most nearly approximates. Stratum, Kimmeridge clay : locality, 

 Boulogne. 



10. C. beamnonti. —Maxillary plate, right upper jaw : length of 

 the outer margin 3 inches 5 lines ; length of the inner ditto 2 inches 

 8 lines ; breadth at the base 1 inch 6 lines ; depth of the symphysis 

 5 lines ; breadth of the principal tubercle 6 lines ; the upper surface 

 is marked by a deep sulcus, 7 lines in width, running parallel with 

 the symphysis ; the inner surface has four triturating prominences, 

 one anterior, two basal, and one intermediate. Stratum, Kimme- 

 ridge clay : locality, Boulogne. 



11. C. dufrenoyi. — Maxillary plate, left lower jaw : length 2 inches 

 4 lines ; depth at the symphysis 1 inch 1 line ; ditto at the medial 

 angle of the dental edge 1 inch 4 lines ; dental edge 1 inch 5 lines ; 

 anterior division of ditto 7 lines ; breadth of ditto 5 lines ; posterior 

 division of ditto 8 lines ; heel 1 inch ; exterior slightly concave and 

 uneven ; inner surface contracts rapidly in diameter in the direction 

 of the base ; anterior tubercle 1 inch 6 lines in length by 6 lines in 

 breadth, placed very obliquely ; posterior tubercle small and narrow. 

 Stratum, Kimmeridge : locality, Boulogne. 



The author then enters into a detailed comparison of the fossil 

 Chimseroids with the recent genera CMma:ra and Callorhynchus, and 

 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 {larx^^ robur, odovs 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 laminee 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 ; 



M 2 



