356 PALAEONTOLOGY OF ILLINOIS. 



closely allied form of the above genus known from American horizons. 

 Tbe present form is distinguished from that last described from the St. 

 Louis, M. occidentalis, by its stouter proportions, strongly arched out- 

 line, the obliquity of the deep face of the base, and relatively shallower 

 opposite face, and more obliquely beveled inferior surface. The coronal 

 region is very similar to the above mentioned form, the pectinations, 

 perhaps, are more tumid and the crest slightly acuminate. 



There remains to be noticed another form with which the present 

 species is'closely allied. This is represented by small teeth occurring in 

 the Mountain limestone at Armagh, Ireland, specimens of which, with a 

 rare suite of other forms, had been borrowed by Prof. Agassiz from the 

 collection of Lord Enntsklllen, which we were so fortunate as to be per- 

 mitted to examine at the museum in Cambridge before they were returned 

 to Florence Court. These teeth are but little larger than the American 

 form, from which they mainly differ in the less vertically arched and 

 laterally curved outline, and, perhaps, fewer pectinations, the median 

 one of which is perceptibly more strongly produced and elliptically 

 pointed^at the apex, than is the case in the present form. 



Position and locality : Upper nsh-bed of the Chester limestone ; Ches- 

 ter, Illinois. 



Genus CHOMATODUS, Agassiz. 

 Choblatodtjs compttjs, St. J. and W. 



PI. X, Fig. 19-22. 



Teeth attain large size, strong, variable in general proportions, and 

 presenting at least two somewhat distinctly marked varieties. One of 

 these varieties is represented by teeth which are mainly distinguished 

 by their abbreviated lateral extremities, which are sharply jounded, and 

 traversed by an obtuse though well-defined median crest, from which 

 both sides alike rapidly slope to the abruptly constricted basal margins; 

 the median portion of the crown is broadly produced in front and 

 behind, and rises into a very strong, prominent, obtusely conical median 

 cone whose apex overhangs the concave face, which is gently concave 

 vertically, the opposite face presenting a gibbose prominence gradually 

 expanding below and distinctly defined from the lateral wings ; the 

 basal margin is regularly curved along the concave side, very slightly 

 arched upward in the middle and gently rounded to the extremities; in 

 the opposite face it is suddenly interrupted in the median region by a 

 more or less deeply notched border, in both faces occupied by a narrow, 

 more or less obscurely imbricated coronal belt, from which arise irregu- 

 lar oblique lines which produce an exceedingly delicate and highly 



