FOSSILS OF THE KEOKUK GROUP. 517 



it is distinguished by its smaller size, narrower and straighter form 

 (particularly at maturity), less incurved beak, prominent anterior ridge, 

 deeper anterior sinus, and proportionally less expanded aperture. It 

 also wants the anterior lateral sinuses of the lip seen in that species. 



It is quite evident that the sinuosities of the lip in shells of this genus, 

 although, as elsewhere suggested, to a considerable extent modified by 

 the inequalities of the surfaces to which the animal had attached itself, 

 still generally show some tendency to regularity in different individuals 

 of the same species, especially in those usually with one or more deep 

 sinu- 



Locality and position. Keokuk limestone of the Lower Carboniferous 

 series, at Xauvoo. Illinois. 



PLATYCERAS rsruxDiBULUM, ]M. and W. 



PL 17, Fig. 3. 



Platyeera* tubrectvm, HALL, 1860. Snpp. Iowa Report, p 1 , (not P. mbrcttum. Hall, 1859.) Twelfth 

 Ann. Rep. Regents' Univ., X. Y., on State Cab., K. H., p. 18. 



Platyeera* (Orthonyehia) infundibulum, MEEK and WORTHKX, 1866. Proceed. Acad. Xat. Sci., Phila., 

 p. 266. 



SHELL straight, more or less elongate-conical, very 

 slightly oblique, somewhat attenuate near the straight, 

 suit-central apex, thence expanding at first rather gradu- 

 ally and then more rapidly to the regularly sub circular, 

 sub-quadrate, or more or less oval aperture; lip rather thin 

 and irregularly undulated, as if to correspond to an uneven 

 surface of attachment. Surface with more or less distinct, 

 undulating, concentric strhe, and near the lip stronger marks 

 or laminse of growth ; also generally provided with a few 

 large, obscure, irregular, undefined longitudinal folds. 



Length, 1.40 inch; breadth, about 1.30 inch. 



As remarked by Prof. HALL, this species varies considerably in the 

 degree of expansion, some specimens being much more attenuated 

 than others. It is possible that in very young individuals, the minute 

 immediate apex may have been curved or sub-spiral, but in all those 

 we have seen it is straight, and sometimes a little compressed, and only 

 removed from the central position by the slight general obliquity of the 

 whole shell, without any curve. In some of its forms it resembles 

 P. Quincyenifis. of McChesney. from the Burlington division of the 

 Lower Carboniferous, though it differs, even when, as is sometimes the 



