TRENTCW LIMESTONE. 173 



This species usually occurs as casts of the interior, the shell being rarely preserved. I 

 have seen a single specimen in which some remains of the shell can be distinguished ; and 

 there is also an obscure indication of the spiral band on the angular margin of the last 

 volution, but the details cannot be made out. 



This species is probably the same as that described by Mr. Sowekby, cited above ; and 

 its similar geological position is a further reason to regard it as identical. It possesses the 

 essential characters of Pleurotomahia, which are better seen in the analogous succeeding 

 species. 



This species is regarded by M. de Verneuil as identical with Euomphalus qualteriatus, 

 cited above ; but it appears to me that there are some reasons for separating it. That 

 species, judging from the figures of Schlotheim, Wahlenberg, Hisinger, Goldfuss and 

 DE Verneuu-, is less depressed than our shell, and the last volution below the angle more 

 vertical and ventricose, giving the aperture a different form as represented by the authors 

 quoted, that of Wahlenberg approaching more nearly to our shell, but being insufficiently 

 extended transversely. In our specimens, the extent of the aperture, from the suture with 

 the next volution, is greater than the vertical height ; while in the figures cited, except that 

 of Hisinger, the vertical diameter is equal to, or greater than, the transverse, and, in that 

 of Goldfuss, nearly twice the transverse extent from the suture to the outer angle. Our 

 specimens are very uniform in character, and we can scarcely conceive such a change to 

 have taken place in the same species on the other side of the Atlantic. The succeeding 

 species bears a more close resemblance to the E. qualteriatus, in the form of the aperture 

 and expansion of the last volution. 



Fig. 6 a. View of the spire (the specimen is a cast). 



Fig. 6 b. Lateral view, shewing the elevation of the spire. 



Fig. 6 e. Base of a smaller specimen (a cast), showing the umbilicus. 



Fig. 6 d Front view of the same, showing the aperture. 



Position and locality. This fossil is more abundant in the higher crystalline portions of 

 the rock at Walertown, than at any other locality. It is rarely found at Middleville, and 

 other localities in the Mohawk valley. At the first named locality, it is more abundant than 

 any other univalve in the rock, except the Bellerophon bilobatus. {State Collection.) 



224. 12. PLEUROTOMARIA ROTULOIDES (m. SJ9.). 



Pi.. XXXVII. Frgs. 7 a, b, c. 

 Compare Euomphaliu gualieriattu, and synonymy as in the preceding species. 

 Depressed conical ; spire composed of about four volutions ; height about equal to half 

 the width of the shell ; outer volution angular at tlie margin, ventricose below and slightly 

 concave above ; upper ones becoming convex above, and slightly elevated vertically at the 

 suture ; umbilicus small ; aperture subquadrate ; surface marked by distinct sharp striae, 

 which curve gently backwards from the suture above ; edge of the outer volution distinctly 

 marked by a spiral band, with abruptly curving strife, indicating the marginal slit. 



