NATURAL SCIENCES OF PHILADELPHIA. 265 



It is quite evident that the nature and position of the sinuosities of the lip, 

 as already suggested, in all the species of this genus, were modified to a con- 

 siderable extent by the nature of the surface to which the animal was attached 

 during life. A careful examination, however, of large collections of most of 

 the known American palaeozoic species, shows that there was generally a ten- 

 dency towards a uniformity in the sinus and the corresponding longitudinal 

 ridges, when present, iu each species, particularly in those of Carboniferous 

 age. 



Locality and position. Keokuk limestone, of the Subcarboniferous series, at 

 Nauvoo, 111. 



Platyceras (Orthonychia) chestere.vse, M. & W. 



Shell small, obliquely conical, more or less arched ; apex sometimes 

 rather attenuate, curved or directed backwards so as to stand nearly over the 

 posterior margin ; anterior and lateral sides expanding rather rapidly from 

 near the apex; aperture irregularly subcircnlar ; lip margin more or less 

 undulated. Surface marked by fine undulating concentric stria? of growth, 

 and usually with about five rather broad radiating furrows that extend from 

 the lip to the middle or above, so as to leave between them five broad obtuse 

 ridges, which are themselves sometimes faintly divided into smaller irregular 

 costae near the margin of the lip. 



Height, or length, measuring from the anterior basal margin obliquely to the 

 apex, O-b'b' inch ; transverse breadth of aperture, 0-55 inch; longitudinal do. 

 of same, 0.53 inch. 



The most marked feature about this little shell is the rather general pres- 

 ence of about five radiating furrows extending sometimes from near the apex 

 to the margin, so as to divide the surface into about five broad ridges, some- 

 times themselves faintly subdivided. This character is not in all cases dis- 

 tinctly defined, though the specimens generally show indications of it, while 

 in some instances it is a very conspicuous feature. In this character it is much 

 like the Burlington group species, P. quincyense, of McChesney, from which, 

 however, it is distinguished by its much smaller size, more rapid expansion 

 and more arcuate oblique form. 



It is a specimen apparently of this species to which we have already alluded 

 as being attached to the side of a Pentremites Godoni. The individual so at- 

 tached is less attenuate, and has the five furrows less defined than the typical 

 specimens of the species, but it nevertheless seems to belong to this species. 



Locality and position. Chester division of the Subcarboniferous series, 

 Chester, Illinois ; also in same position Pope county, 111. 



PLATYCERAS (OrTHONYCHIA) SUBPLICATUM, M. & W. 



Shell small, depressed conical, somewhat oblique, rapidly expanding from 

 a subcentral apex; anterior slope slightly convex; posterior and lateral 

 slopes straight or a little concave ; aperture subcircular ; adductor muscular 

 scars finely striated and placed a little above the middle on each side ; elongate- 

 subovate or sublunate, being a little arched, with the larger end forward and 

 raised slightly higher than the other, and the posterior ends connected by a 

 linear depression running around behind ; surface (of casts) with a few large, 

 irregular radiating folds or plications extending from the margins of the aper- 

 ture more than half way up towards the apex. 



Height, 0'36 inch ; anteroposterior diameter, 063 inch ; transverse, do., 

 05b' inch. 



We have only seen internal casts of this species, which probably do not give 

 a very correct idea of the nature of the apex, which in the casts is rather ob- 

 tuse and merely directed somewhat obliquely backwards and upwards. In 

 perfect shells it is doubtless pointed and more or less incurved. The plications 

 of the surface are obtuse and rather obscure in the internal casts. The sur- 



1866.] 



