YEETEBEATES. 295 



behind, principal prominence median and vertical, or eccentric and lat- 

 erally deflected, the apex directed towards the short extremity, recurved, 

 strongly buttressed in front, the vertical ridge terminating in the basal 

 protuberance below, lateral wings sharp-crested, and bearing a strong 

 secondary cusp at the extremities, which is sometimes flanked exteriorly 

 by a smaller denticle, one or more similar denticles between the lateral 

 cnsps and the median prominence; in the perfect condition, these deu- 

 ticulations are observed to be produced by the convergence in the crest 

 of one or two relatively strong, vertical ridges, which appear in both 

 faces, though more conspicuous in front, the median buttress forming a 

 prominent, angular ridge, which is perhaps ornamented with delicate 

 thread-like lines. A specimen of medium size measures . 10 inch in lat- 

 eral diameter, greatest hight .06, anteroposterior breadth of base . 04 

 inch. 



The present form also presents considerable variability, though per- 

 haps to a less extent than noticed in the preceding forms, and which 

 would appear to be largely due to the state of preservation of the indi- 

 vidual teeth. Our material contains several examples some of which 

 are larger than the typical individuals, but which are perfectly smooth, 

 simply exhibiting the general coronal contour, the crest, median and 

 lateral prominences obtusely rounded — features doubtless attributable 

 to abrasion, as they otherwise differ in no respect from the typical 

 examples. The form recalls that previously mentioned under the name 

 M. explanatus, with which, indeed, it bears resemblances in general out- 

 line and proportions; but the coronal ridges are stronger, and the ante- 

 rior face of the median cusp is much more prominently buttressed. A 

 specimen obtained by one of the writers fioin the Keokuk limestone, 

 though not perfect, exhibits no appreciable differences by which it can 

 be distinguished from the present form. 



Position and locality : Xot rare in the fish-bed of the Upper Burling- 

 ton limestone ; Buffiugton creek, Louisa county, and at other localities 

 in the same horizon, in Illinois and Iowa. 



Genus OROD US, Agassiz. 

 Oeodtjs ? paealleltjs, St. J. and W. 



PL V, Kg. 23. 



Teeth very small, occurring in serial rows, apparently soldered at their 

 bases, which thus form a common support, probably also occurring 

 isolated, stoutly built, and symmetrical. Individually, the base is very 

 strong, very obliquely produced backward, anterior face deeply chan- 

 neled and defined from the crown by a narrow, sharply projecting hori- 



