NATURAL SCIENCES OF PHILADELPHIA. 61 



Length of a medium sized specimen, 0-92 inch ; breadth of do.. 1-20 inch 

 convexity, 0-72 inch. 



As nearly as can be determined from a description onlv, this shell would 

 seem to be closely related to S. macrothyris, Hall, from the Upper Helderberg 

 Limestone of New York and Ohio (see 10th Report Regents, p. 133), but differs 

 in being always narrower in proportion to length, never being near " twice as 

 broad as long." Its area also differs in narrowing regularly to the lateral ex- 

 tremities of the hinge, instead of having parallel margins. On comparison, 

 with good specimens of S. Oweni, Hall, from the Upper Helderberg Limestone at 

 the falls of the Ohio, which species our shell nearly resembles, it is found to 

 differ in having the beak and area of its ventral valve always more strongly 

 arched. Its area is also proportionally narrower, and its plications larger and 

 less numerous. There are in the collection a large number of well preserved 

 specimens, showing the characters given to be very constant. 



Locality and position. Pinon Station, Nevada. Devonian, probably of the 

 age of the Upper Helderberg Limestones of New York. 



Lower Silurian Species. 

 Euomphalus (Raphistoma?) rotuliformis, Meek. 



Shell small, sublenticular, or more than twice and a half as wide as high, 

 with the periphery sharply angular, and the much depressed spire a little 

 more prominent than the convexity of the last turn below the angular peri- 

 phery ; umbilicus very wide, deep, and depressed conical ; volutions six or 

 seven, exceedingly narrow, and increasing very gradually in size, all obliquely 

 flattened, or sometimes slightly concave on the upper slope, which is nearly 

 coincident with that of the spire, and with the under side sloping downward 

 and inward, and nearly one-third wider than the upper surface to the umbili- 

 cus, around which they are rather distinctly angular; aperture obliquely 

 rhombic. Surface unknown. 



Breadth, 0-32 inch; height, 0-12 inch; breadth of last turn on the upper 

 slope, 0-15 inch ; do. on the under slope, 0-18 inch ; breadth of aperture, 0-09 

 inch ; height of do., 0-07 inch. 



This species is evidently nearly allied to Euomphalus polygyratus, Roemer, 

 from the Lower Silurian rocks of San Saba, Texas (see Kreid. Von Texas, 

 tab. xi, fig. 4 a, b). It differs, however, in being much smaller, its greatest 

 diameter being less than one-fourth that of Roemer's species, although it 

 shows nearly the same number of volutions. Its volutions are also propor- 

 tionally more convex below, and slope more abruptly into the umbilicus. 



Locality and position. Ridge south of Muddy Creek, Nevada Territory, from 

 a gray subcrystalline limestone of Lower Silurian age, probably of the same 

 horizon as the Calciferous sand rock of the New York series. 



Euomphalus (Raphistoma?) trochiscus, Meek. 



Shell sublenticular, about twice and a half as wide as high, spire much de- 

 pressed, or but little higher, measuring from the horizon of the sharply angu- 

 lar periphery, than the convexity of the last turn below the same ; umbilicus 

 wide, deep and depressed conical ; volutions four and a half to five, increas- 

 ing gradually in size, all obliquely flattened (or sometimes slightly concave) 

 above, nearly on a line with the slope of the spire, and sloping downward and 

 inward below to the umbilicus, into which the curve is so abrupt as to form 

 an obtuse angle around the same ; aperture wider than high, and rhombic 

 subtrigonal in outline. Surface unknown. 



Breadth, 0-40 inch; height, 0-15 inch; breadth of last turn, 012 inch; 

 breadth of umbilicus, about 0-25 inch. 



This is similar in general appearance to the last species, but may be read- 

 ily distinguished by its less numerous whorls, which increase more rapidly in 



1870.] 



