VERTEBRATES. 307 



their associates. In the depth aud excavation of the outer basal region 

 there is strong resemblance to 0. tuberculatus, K. and W.; but the 

 coronal portion is so markedly different as to render their identity with 

 that species hardly probable, and the peculiar pyramidally produced 

 apex and strougly arched contour of the crown afford characters which 

 may be relied upon in the determination of the identity of the individ- 

 uals. 



Associated with the above mentioned teeth, there are a few individ- 

 uals of the same general form, but which have their crests reduced to 

 the condition of a smooth arched plate — indeed such as might be pro- 

 duced by the wearing away of the crown to a line little above the basal 

 margin. The individual variations of the latter teeth are almost as 

 striking as are the differences by which they are distinguished from the 

 teeth above described, and with which they are probably specifically 

 identical. 



The Keokuk limestone fish-bed horizon also affords teeth which some- 

 what closely resemble the present form, but which are in too imperfect 

 condition for satisfactory determination. 



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

 ton limestone, Louisa and DesMoines counties, Iowa. 



Orodus carinatus, St. J. and W. 



PI. V, Fig. 24. 



We have from the Keokuk limestone a couple specimens of a peculiar 

 form of Orodus, which seems to be distinct from those with which it is 

 associated at the same locality, and which are distinguishable by the 

 following characteristics: Teeth of small size, a medium sized speci- 

 men measuring in lateral diameter .13 inch, and in hight nearly .05. Base 

 thick, obliquely produced behind, more or less arched vertically, anterior 

 face moderately channeled, inferior surface relatively broad; both ante- 

 rior and posterior faces marked by rather coarse vertical grooves or 

 pits. The crown forms a very compressed, gently and regularly arched 

 sharp-crested prominence, slightly overreaching the lateral borders of 

 the base, and constricted before and behind, and occupied in either 

 face by strong, buttress-like, laterally deflected carinse, which culminate 

 in the crest and give rise to a series of delicate tuberculations in the 

 lateral portions ; the posterior basal portion of the crown is further 

 delicately ornamented by the peculiarly interrupted plicse, such as char- 

 acterize certain species of the genus, but the character of the opposite 

 basal border is not satisfactorily exhibited by the specimens before us. 

 The crown is remarkably thin, resembling in this respect Chomatodus, 

 but its vertically ridged surfaces clearly indicate its generic identity 

 with Orodus. 



