VEKTEBRATES. 221 



sentatives of the present form occurring in the St. Louis limestone, 

 vary considerably one from the other. The large, normally entire 

 tooth in the collection of Mr. Van Home is supplemented from the 

 same locality by a proportionately shorter tooth, which has the 

 angulation of the inner articular border situated nearer the middle 

 of the tooth instead of one-fourth the distance from the anterior 

 extremity as in the specimen just mentioned, with which it exactly 

 agrees in other particulars. A third example from the same locality, 

 at Alton, offers still other differences, as in the obliquity of the 

 anterior margin, the angulation of the inner border being crowded 

 forward and merged with the inner lateral angle, the stronger develope- 

 ment of the outer basal rim, and the prominent transverse rugae 

 that envelope the posterior two-thirds of the coronal surface, the 

 anterior slope and the ridge parallel with the inner border being 

 worn smooth, revealing the ordinary punctate structure noted in the 

 above mentioned specimens. A smaller tooth derived from the same 

 formation, at Pella, differs from the above in the very slight 

 developeinent of the angulation in the articular border, which is 

 situated about one-third the distance from the anterior angle ; the 

 corona] fold also shows a sharp angulation along the vertical inner 

 border, but the opposite border is worn away, destroying every 

 vestige of the prominent basal rim usually occurring along that side 

 of the tooth, though the basal region is otherwise very like that 

 described from normally preserved specimens. The coronal surface, 

 however, is worn quite smooth, showing the ordinary puncta3. We 

 are inclined to regard all of the above mentioned specimens as 

 probably pertaining to one and the same species, the variable con- 

 dition of the coronal surface being due to the effects of trituration, 

 and the other peculiarities, as the position of the angulation in the 

 inner border, such as may be attributed to individual variation. 

 This inference, however, may appear inconsistent at first sight, but 

 we have already observed marked variations in the same parts, as 

 noted in individuals described under Psammodus plenus, of whose 

 specific identity scarcely a doubt can arise. 



The specimen figured in PL XI, f. 3, 3a, of Vol. II., 111. Geol. 

 Survey, where the form is referred by Messrs. Newberry and Wor- 

 then to Psammodus rugosus, Agass., also represents a short tooth, 

 in an almost perfect state of preservation, save the wearing away 

 of a patch of the crown at the outer anterior angle. It presents a 

 rhomboidal outline, the anterior margin gently curved forward, the 

 deep basal portion inbeveled, the posterior margin correspondingly 



