294 DESCRIPTION OF A COLLECTION OF FOSSILS, 



about 15 or 16 large ribs; angular on top and sloping on the sides; interspaces 

 angular at the bottom ; on the sides of the ribs are faint traces of two radiating 

 lines ; all crossed by minute, subsquamose lines of growth. 



Diameter about 1.3 inch. 



Locality. From the " hill of San Antonio, with silver mines, Morococho ; alti- 

 tude 4500 metres," associated with Rhpichonella Antonii. No geological age given. 



Remarks. I have ventured to name this shell although the outline is almost 

 entirely destroyed, since it cannot fail to be recognized by its slightly convex shell 

 and the peculiar angular ribs and interspaces, a cross section of which forms a 

 zigzag line. 



NEITHBA, Drouet. 



N. QUiNQUECOSTATA, Sowerbv, sp. 



Peden id., Sby., Min. Conch., PL 56. 



Janira id., d'Orb., Pal. Fr., Ters. Cret., p. 632, PL 444, f. 1-5. 



Neithea id, Gabb, Synopsis Cret. MolL, 1861, p. 148. 



A single small specimen, about half grown, retaining both valves. It agrees 

 fully with the European specimens, in all the details of form and ribs. Each large 

 rib has a smaller one on its lateral slope, and-the space between each pair of large 

 ribs is occupied by two of slightly smaller size. The specific distinctions in this 

 genus rest on comparatively trivial characters, in the details of surface ornament, 

 and I should have hesitated in referring this shell to Sowerby's species, had I not 

 compared it with authentic specimens from England and France. 



Locality. Elevated table-land two leagues from Cajamarca. Cretaceous. 



Remarks. We have here an excellent key for establishing a geological horizon, 

 in the presence of a well-known upper greensand species; although it does not 

 follow by any means that the two deposits were absolutely synchronous in their 

 age of deposition. The species may have originated in either of the two regions, 

 and have emigrated, becoming extinct in one before it made its appearance in the 

 other. That this does occur with animals as well as plants is a well-recognized 

 fact; nevertheless, the presence of a species whose geological horizon is estab- 

 lished, gives us the means of fixing approximately the age of any new deposit in 

 which it may be discovered. 



N. ALATA, Von Buch (sp.). 



Pecten alatus, Von Buch, Petr. rec. en Araer., p. 3, PL 1, f. 1-4. 



P. Dufrenoyi, d'Orb., Am. Mer., p. 106, PL 22, f. 5-t. 



P. alatus, Bayle & Coq., Mem. Soc. Geol. Fr., voL 4, p. 14, PL 5, f. 1-2. 



P. Dtt/reno2/i, Hup.; Gay's Hist, de Chili, p. 291. 



Janira alala, Ecmond; Pal. de Chili (pamph.), p. 18. 



Neithea alata, Gabb, Synopsis Cret. Moll., p. 147. 



