322 PROCEEDINGS OF THE ACADEMY OP 



gins broken awa3^ The beak of its right valve projects rather 

 decidedly above that of the left; but I think this is due to acci- 

 dental displacement of the valves, rather than to any inequality 

 in their size. It shows distinct indications of a well-defined, mo- 

 derately wide cardinal area, widest under the beaks, and narrow- 

 ing to the extremities of the hinge. 



Mr. James referred this species, in his list of the Cincinnati 

 fossils, with a mark of doubt, to the Lower Helderberg species, 

 31. Sinnneri of Hall. But, in addition to the rather widely dif- 

 ferent o-eoloofical horizons from which these two shells were ob- 

 taiued, they seem to me to ditfer so materially in form as to be 

 clearly distinct species, even if similarly marked, while the typi- 

 cal specimen of M. Spinneri shows no traces of the regular 

 radiating costre seen on the species here described. It is true 

 that the specimen of that species figured is an internal cast, and 

 ours a cast of the exterior, which might account for the difference 

 of surface characters, but this would not produce the degree of 

 difterence in form, obliquity, and general physiognomy. To me, it 

 appears to be much more nearly like the typical species M. carclii- 

 formis, from the New York Upper Helderberg limestone, though 

 clearly distinct in having much larger costse as well as a wider 

 and more defined cardinal area. 



The group Ifegambonia of Hall, 1859, seems, so far as 3^et 

 known, scared}' more than subgenerically distinct from the typi- 

 cal forms of Gyp7'icardifes ; and Mr. Billings thinks it exacth'" 

 agrees with the group for which he proposed the name Va7ii(xe- 

 mia, in 1855, and placed as a subgenus under his genus Cyrtodonta, 

 1858, a species of which is the type of Conrad's Gypricai'dites, 

 ISil. If the name Vanuxeniia should be retained for the type 

 under consideration, and that group placed as a subgenus, then 

 the name of our fossil, when written in full, would be Gypricar- 

 dites (Vanuxemia) Jamesi ; but, if Megamhonia is distinct from 

 Vanuxemia, and a subgenus under Gypricardites^ then its full 

 name would be Gypriccu^dites {Megamhonia) Jamesi. 



Locality and position. — Cincinnati group of the Lower Silurian, 

 at Cincinnati, Ohio, about 350 feet above low-water mark of the 

 Ohio. Collection of Mr. U. P. James's, in honor of whom the 

 species is named. 



[February 13, 



