VERTEBRATES. 67 



upon the central one. The root has, on the inner side, an 

 altitude one-third less than that of the crown, there presenting 

 • a vertical, coarse and porous surface. From the lower border 

 of this the inferior surface slopes up to the corner on the oppo- 

 site side. 



In size and general character the species may be compared with the two pre- 

 ceding, but in the details of its structure is still quite different from either. 

 From 0. ornatus it may be at once distinguished by its very distinct lateral 

 cones, which are separated by deep sulci, and, like the middle one, are con- 

 stricted at the base; the ridges coarser and more obtuse, and more restricted to 

 the base of the crown. 



From 0. elcgantulus it may be easily separated by noticing its more conspicu- 

 ous lateral cones — its stronger ornamentation occupying the base of the crown 

 on either side, while in 0. elegantulus it is mainly restricted to the summit and 

 one face. 



In the specimens contained in the collection we have one on which are more 

 than a dozen teeth, that apparently belonged to a single individual. It is 

 therefore of special interest as affording a little of the information so rarely 

 obtained in reference to the variety of form displayed in the dental series of a 

 single plseozoic fish. Unfortunately this specimen is much weathered, and all 

 finer markings are oblitered. We can see, however, that there was considera- 

 ble diversity of size and some difference of form among them, there being longer 

 and shorter ones, those nearly straight and those strongly arched, those with 

 conspicuous roots and those with little or none, and finally those relatively thick 

 and strong and others very slender. The surface marking is, however, as far 

 as it has been preserved, essentially the same in all. Many of these teeth were 

 apparently much worn during the life of their possessor, and it is evident from 

 their condition, as well as from other facts which have come under our obser- 

 vation, that the subsistence of Oroclus was made up, for the most part, of mol- 

 lusks or other resistant substances, which it was necessary to crush, and in 

 triturating which their teeth were much worn and impaired. 



Figures 10 and 10 a, side and top views, natural size. 



Formation and locality : Keokuk limestone, Nauvoo and Warsaw, Illinois. 



Orodus minusculus, N. and W. 



PI. IV, Fig. 11. 



Teeth very small, arranged in a series of five or more, dimin- 

 shing in size from front to rear, their summits forming an arch 



