212 PALAEONTOLOGY OF ILLINOIS. 



Mandibular teeth subquadrangular in outline, rather strongly 

 arched from within outward. Lateral borders unknown. The ante- 

 rior and posterior margins marked by a narrow coronal fold well 

 denned from the basal portion, which latter in front is slightly 

 channeled with a corresponding convexity along the posterior mar- 

 gin. The coronal region, besides the rather strong longitudinal 

 arching, presents a gentle transverse convexity, which probably 

 occupies the greater extent of the coronal area. The anterior half 

 or more of the surface is occupied by a transverse prominence, 

 behind which the surface steeply slopes to a wide plane area bor- 

 dering the posterior margin, presenting a contour somewhat cor- 

 responding to that of mature examples of the opposite jaw. The 

 rugose ornamentation and punctation of the coronal surface presents 

 a repetition of the characters noted in connection with the maxillary 

 form. The imperfect sole representative of the form shows a longi- 

 tudinal diameter of 90 millimetres, indicating the huge dimensions 

 attained by the fishes represented by the present teeth, and which 

 was, so far as w r e are aware, the largest of its kind. 



The above species is known to us from a couple of imperfect teeth 

 referable to the upper jaw, and a narrow section of a gigantic speci- 

 men belonging to the opposite jaw or mandible. The smaller of the 

 maxillary teeth shows scarcely a vestige of the rugose coronal orna- 

 mentation, the surface on the contrary being densely occupied by 

 the exposed orifices of the vertical tubes, which exhibit various stages 

 of wear, from the simple puncta3 of excessively abraded areas to the 

 crater-like form surrounded by a rim of intensely hard substance, 

 the puncta3 simple or confluent, with discernible traces of the coarser 

 prismatic structure. We are inclined to regard this tooth, at first 

 sight so dissimilar in its superficial aspects, as specifically identical 

 with its associates. It is an extremely rare species, and we regret 

 the lack of materials necessary for a comparison with the forms of 

 the typical European species with which it shares many points of 

 resemblance. 



Geological position and localities: Keokuk limestone, Keokuk and 

 Bentonsport, Iowa. 



