VERTEBRATES. 127 



along antero-lateral border to which its anterior slope is steeply 

 beveled. Enamel coating along inner margin showing faint lines of 

 growth, the worn crown surface exhibiting relatively coarse crowded 

 puncta3. Length of antero-lateral border to point of inrollment 1.5 

 mm. ; breadth across inner margin, 2.5 mm. 



The above form is represented by a single example, and notwith- 

 standing its minute size, the perfect state of preservation of the 

 tooth permits a detailed interpretation of its diagnostic charac- 

 teristics and comparison with allied forms. We are, however, in 

 some doubt as to its generic identity, although there is strong evi- 

 dence of its intimate relationship with Cochliodus, the Strebloidal 

 maxillary posterior form of which it closely resembles in coronal 

 contour. This unique specimen, however, does not seem to be 

 referable to the associated species Coch. Van Hornii, unless we are 

 to suppose a most extraordinary change takes place, by which young 

 and mature teeth are made to assume the most unlike semblance 

 to one another. The present tooth, however, compared with the 

 streblodoid maxillary form of Coch. Van Hornii, is distinguished by 

 the following pronounced contrasts : the general obliquity of outline 

 and relatively narrower breadth compared with the length in the 

 direction of inrollment (a feature which is necessarily persistent and 

 directly dependent on the Jaw of development of the dental plates), 

 the extremely prominent posterior lobe, and the depressed median 

 portion of the neck denned by the comparatively prominent ridge 

 along the antero-lateral border. It is in the latter respects the form 

 here especially alluded to offers greatest contrast with the prevailing 

 coronal contour of Cochliodus, and suggests a possible relationship 

 with Deltoptychius. 



Geological position and locality: St. Louis limestone ; St. Louis, 

 Missouri. 



COCHLIODUS LEIDYI, St. J. and W. 



PI. VII. Fig. 11-16. 



Teeth of small size, the collections furnishing representatives of 

 the posterior and median teeth of the mandible, and the posterior 

 form and Helodus-like series of teeth of the upper jaw. 



Mandibular posterior teeth distinguished by their compact build and 

 relatively narrow transverse diameter compared with the length in 

 the direction of the strong enrollment. Antero-lateral border slightly 

 curved between the inner angle and point of inrollment, presenting 



