NATURAL SCIENCES OP PHILADELPHIA. 13 5 



Helix occidkntalis. 



Shell thin, rather small, depressed nearly orbicular; volutions four, convex 

 above, proniiuent, or almost subangular a little above the middle of the outside, 

 gibbous below, most convex near the umbilicus ; surface ornamented by regular 

 rather strong oblique lines of growth; suture well defined; umbilicus deep, less 

 than one third the transverse diameter of the shell, exhibiting most of the inner 

 volutions ; aperture round oval, modified slightly by the succeeding whorl ; lip 

 sharp, not reflexed. Greatest transverse diameter .33 inch; height .21 inch ; 

 greatest diameter of aperture .16 inch. 



This neat little Helix is more nearly related to the recent H. strialella of An- 

 thony than to any other shell we have seen figured, but is larger, has a 

 smaller umbilicus, and appears to be more strongly striated. iMr. Anthony says 

 the umbilicus of IT. striateUa is not equal to the body whorl. In the species 

 before us, it is scarcely half equal to the body volution near the aperture. 



Locality and position. Same as last. 



Helix vitrinoides. 



Shell subglobose; spire elevated; volutions about four, convex, increasing 

 rather rapidly, last one somewhat ventricose, contracted a little near the lip ; 

 suture distinct; umbilicus very small or closed; aperture oval or ovate; 

 (lip reflexed?) ; surface unknown. Height .37 inch ; breadth .44 inch ; greater 

 diameter of aperture .27 inch ; smaller do. .20 inch. 



Of this species wc have only seen internal casts, and consequently know 

 nothing of its surface markings. Judging from the slight contraction of the 

 body whorl of the cast near the aperture, it is probable the lip was thickened 

 and reflexed. 



Locality atid position. Same as last. 



Planorbis tenuivolvis. 



Shell planiorbicular ; spire flat or nearly so ; volutions six to six and a 

 half, extremely narrow and closely embracing ; increasing very gradually, 

 scarcely one-third as broad as high, rounded above, prominent, or almost 

 subangular round the outer margin, from which they round obliquely down- 

 wards and inwards towards the umbilicus, near which they are more prominent 

 or subangular; suture well defined; umbilicus deep, acutely conical ; equal- 

 ling about one-third the greater diameter of the shell, and exhibiting the vo- 

 lutions quite to the apex of the spire ; surface and aperture unknown ; section 

 of volutions extremely narrow and strongly arcuate. Greater diameter .15 

 inch ; height .OG inch. 



This remarkably delicate little shell resembles the following species (P. am- 

 plexus) more than any with which we are acquainted ; it differs, however, in 

 having much narrower and more closely wound volutions ; about the same 

 number being present in specimens measuring not much more than one-third 

 the transverse diameter of that shell. 



Locality aiid position. Same as last. 



Planorbis amplexus. 



Shell planorbicular ; spire flat, or nearly so ; volutions six, closely em- 

 bracing, narrow, very gradually increasing, nearly twice as high as wide, 

 rounded on the outside, subangular round the umbilicus ; surface marked by 

 lines of growth, which pass round very obliquely backwards from above to a 

 point below the middle of the outer part of the whorls, where they are again 

 directed slightly forwards, so as to indicate the presence of a broad, undefined 

 sinus, below the middle of the outer lip ; aperture unknown, umbilicus conical, 

 deep, less than half the greater diameter of the shell, exhibiting the inner 

 whorls. Greatest diameter .39 inch ; height .25 inch. 



This species bears some resemblance to our P. convolutus (Proceedings Acad. 

 Nat. Sci. vol. 8, p. 1 20,) but has from one to one and a half more volutions, which 



1857.] 



