NATURAL SCIENCES OF PHILADELPHIA. 



2G3 



Pentremi'te that the fit looks as if it might have been air tight. The corres- 

 ponding undulations of the lines of growth likewise show clearly that this nice 

 adaptation of the margins of the lip to the irregularities of the surface of the 

 Penfremite could not have resulted from accidental pressure when the edge of 

 the lip was somewhat yielding, since these curves in the marks of growth are 

 seen to extend up the sides of the shell some distance from the margin, where 

 there could have been no flexibility. 



This habit of attaching themselves to Crinoids, has led some to think the cri- 

 noids were in the act of devouring these mollusks at the moment when they per- 

 ished, and that these mollusks constituted the chief food of the crinoids. So far as 

 our observations go, however, we do not think the evidence sufficient to es- 

 tablish this fact, since these shells are as often attached to the side of'the crinoid 

 below the horizon of the arms as to the summit, and hence out of reach of the 

 mouth, while the conformity of the margins of the shell to the inequalities of the 

 surface to which they are found attached, rather indicates that they grew there. 

 The probability seems to be, that like various other sedentary marine animals, 

 these mollusks, in their very young state, floated freely about until they found 

 a suitable place to attach themselves. We were at one time inclined to think 

 there might also be some reason for believing that the a lult shell at least some- 

 times changed its station, from the fact that iu some instances we observe the 

 lines of growth indicating strong sinuosities iu the lip during apart of the growth 

 of the shell, which afterwards became suddenly obliterated, to give place to a 

 different set of irregularities, as if the animal had changed its stat on and 

 adapted the sinuosities of its lip to a new surface. This, however, may have 

 been produced by the lateral expansion of the lip, by which it was brought into 

 contact with different inequalities as the shell increased in size. We have no 

 evidence that they possessed the power of excavating a depression in the sur- 

 face of attachment, as in Amalthea, or of secreting a shelly layer or support 

 under the foot, as in Hipponyx. 



Prof. Hall has' proposed to establish two subordinate groups under this 

 genus, more or less distinct from the typical forms of Platyceras. These may 

 be distinguished thus : 



1. Platijciras, Conrad. (Typical.) Shell with apex incurved or spiral ; sur- 

 face concentrically striated, sometimes radiately plicate, rarely spiuiterous. 

 Pileopsis iubifer, Sow. 



2. Orthonychia, Hall. Shell arched or straight, with concentric striae. 

 P.'aiycerns subrectum, Hall. 



3. Igoceras, Hall. Differing from the last in having the surface cancellated. 

 Ex. P. plicatum, Conr. 



It is, however, often very difficult to separate the species into these groups, 

 owing to the numerous gradations by which they bleud into each other. 



PLATYCEKAS LJEVIGATUM, M. & W. 



Shell small, dextral, subglobose, composed of two to two and a-half very 

 rapidly expanding contiguous whorls, the first of which is minute ; last whorl 

 forming much the larger part of the shell, evenly convex, and although in- 

 creasing rapidly in size, not properly campanulate ; aperture nearly circular, 

 being somewhat straightened on the inner side ; lip not sinuous in any 

 of the specimens examined ; surface nearly smooth, but showing fine lines of 

 growth under a lense, where not worn. 



Length, 0-55 inch; breadth, 38 inch. 



This little shell is not very nearly related to any of the other carboniferous 

 species of this country with which we are acquainted. It will be readily 

 identified by its small size, rapidly expanding whorls, smooth surface, without 

 folds or plications, and the non-sinuous, regular outline of its lip. From the 

 latter character, it would seem to have attached itself only to even surfaces. 

 In size and the regular smoothness of its surface it is quite similar to 



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