296 DESCRIPTION OF A COLLECTION OF FOSSILS, 



Avhich had evidently attached themselves to the inner face of the larger one after 

 the death of its occupant. The lower valves are lost in all but one example, and 

 there only traces of it remain. 



PLACTJNANOMIA, Brod. 

 P. (Paranomia?) lima, n. s., PL 42, fig. 7. 



Shell flat, equi valve, subtriangular ; upper margins irregularly sloping ; base 

 broadly rounded; surface covered with rough subsquamose lines of growth, crossed 

 by small radiating ribs which, at short intervals, rise into little spines like the 

 teeth of a rasp, or better, like the low spines of some species of Spondylus. 



Measurements. Length of largest specimen, 2.0 in.; width, 1.7 in. 



Localities. From the neighborhood of Ollon ; the Hacienda of Macango, and 

 Quebraba of Colpamayo, near Chota, Dept. of Cajamarca. Cretaceous. 



Remarks. None of the specimens show the hinge or the internal structure, so 

 that we do not know whether or not it has the internal plate of Conrad's subgenus. 

 It resembles the typical species of that group in the sculpture of the surface, but 

 both valves seem to be equally convex and equally strongly marked with the 

 scabrous ribs. 



OSTREA, Linn. 



0. callacta, Con., PI. 42, fig. 2, 2a. 



id., Con., Proc. Acad. Nat. Sciences, 1875, p. 139, PL 22, f. 1. 

 O., s^. indet, Dana, Wilkes' Bxped. Rep., PL 15, f. 1. 



Shell large, oval, nearly equilateral ; beaks central, small. Lower valve with 

 one large median ridge, from the lower part of which branches another on each 

 side, the terminations of the three occupying the entire basal margin ; a large 

 curved rib runs from the beaks, describing a quarter of a circle, and terminating 

 on the middle of the side ; above this on each side are two smaller ribs, rapidly 

 diminishing in size. Each rib on the lower valve corresponds to an interspace on 

 the upper valve, and each interspace of the lower to a rib on the upper. Entire 

 surface covered by rough lines of growth. Area broad, flat ; ligament pit very 

 broad and shallow, oblique ; margin squamose, not crenulated ; muscular impres- 

 sion moderate in size. 



Measurements. Diameter from beak to base, 4.5 in.; from side to side, 4.0 in. 

 Locality. From the Cretaceous at the Hacienda of Macanga, Prov. of Pataz, 

 Mr. Conrad's type came from " the Pampa del Sacramento, Eastern Peru," and he 

 conjectured it to belong to the " Pebas Group" of brackish water Tertiary, first 

 made known by me in 1868. 



