1870.] -I'^'J [Ilaydoii. 



its breadth, base shai'ply beveled, coronal band narrow, imbrications very 

 obscure or obsolete, gently descending in the middle and slightly cui-ved 

 downward at the lateral extremities; upon the posterior face the base of 

 the crown is defined by a conspicuous transverse ridge, which unites the 

 lateral extremities, and gives origin to a deep central coronal cavity; the 

 enamel-like coating lines the walls of the cavity, and spreading over the 

 gently and regularly downward arched transverse shoulder, it forms a thin 

 coronal band with one or two faint imbrications upon its external inflexed 

 border. The root is nearly as wide and much thicker than the crown, 

 tapering rapidly and rounded at its extremity; anterior side convex or 

 ridged, posterior face slightly concave transversely, both surfaces more or 

 roughened. 



Inches. 



Greatest length, 77 



Greatest breadth at the lateral angles of the crown, 72 



Height of crown upon its anterior face, .32 



Depth of the coronal cavity from the apex of the median 



denticulation, about 45 



And from the transverse shoulder, about 20 



The collection contains a perfect individual of the above described spe- 

 cies, from Bellvue, Nebraska, imbedded in a matrix of limestone, but ex- 

 hibiting the entire posterior aspect of the tooth without a blemish; and I 

 owe to the kindness of Mr. J. Sterling Morton, of Nebraska City, another 

 equally perfect specimen, obtained from a shaft excavation near the City, 

 which shows the anterior face of the tooth. I think there can be no ques- 

 tion as to their specific identity with the form described by Messrs. New- 

 berry and Worthen, from the Upper Coal Measures of Illinois. 



I am acquainted with but a single other form to which this species 

 seems to be closely related, and that is from the mountain limestone of 

 Yorkshire, England. Specimens of the latter species are in the extensive 

 collections of the Museum of Comparative Zoology at Cambridge. The 

 English specimens are, however, markedly specifically distinct from the 

 American; they ai-e less curved laterally, and possess some sharp, thick 

 serrations on either side of the median cusp ; the crown is relatively 

 higher, and the coronal band on the outer face is more deeply arched 

 downward in the middle, is wider and more distinctly imbricated; the 

 coronal cavity of the inner face is shallower, and the transverse shoulder 

 less prominent. I am not aware that the English species is described. 



Fo7'm. and Loc. — Upper Coal Measures. 



Genus CHOMATODUS, Agassiz. 

 Chomatodus aecuattjs, n. sp. 

 A fragment of limestone from Bennet's mill, near Nebraska City, pre- 

 serves the impression of a tooth of the genus Chomatodus, which seems 

 to be distinct from all the species of this genus heretofore described from 

 the Coal Measures and Subcarboniferous. The impression presents almost 

 the entire figure of the anterior face, from which the following description 

 is o'iven: 



