328 PROCEEDINGS OP THE ACADEMY OF 



convexity. Ears unknown. Surface marked by about forty slender, rounded 

 ribs of variable size, often showing a marked tendency to alternation of large 

 and small ones. Interspaces between the ribs flat, and equal to, and some- 

 times wider, than the ribs. The surface, both of ribs and interspaces, is 

 covered by very fine concentric lines placed close together, often hardly 

 visible to the naked eye, and sometimes assuming the form of comparatively 

 large imbrications. Interior of the shell marked by grooves corresponding 

 with the external ribs. 



This species occurs occasionally in the dark-blue marls of Burlington and 

 adjoining counties, but I have never seen a whole shell. It seems to be of the 

 form and size of P. Burlingtonensis nobis, but is sufficiently distinguished by 

 the radiating ribs. The valves are about half an inch apart in the middle. 



P. a s p e r Lam. There are two specimens of this species, in the Academy's 

 Collection, among the American specimens. One purports to have come from 

 Tennessee, the other is without a label. They may both be, however, Eu- 

 ropean specimens, placed there by mistake. I mention the fact to call the 

 attention of collectors to the matter. 



Ostrea Linn. 



0. p a n d Si f o r m i s . Subquadrate. Lower valve very convex, nearly 

 pyramidal, most prominent in the centre, from which point the sides slope 

 abruptly, the anterior half forming nearly a right angle with the line running 

 from the beak to the crest. Laterally, the slope is not so great. Upper valve 

 flat, for the first third of its length, then convexly curved downwards, the 

 anterior half becoming deeply concave in the middle. 



Holding the shell so that the beaks are furthest off, the right hand side i3 

 entire ; the left side is emarginate towards the basal margin. Surface 

 strongly imbricated, the surfaces of the layers being smooth ; no costse visi- 

 ble on the surface. 



Length, 3 in. Width, 2-5 in. Greatest depth, 2 in. 



Locality, " seven miles below Yazoo, Mississippi." My collection. From 

 Dr. Janeway, U. S. A. Most probably cretaceous. 



The irregular form of this shell makes it exceedingly difficult to describe. 

 It resembles, remotely, some of the more regular forms of 0. panda Morton, 

 but can be distinguished by its much greater size, the- deeper inferior valve, 

 the entire absence of ribs, and the strongly imbricated surface. The growth 

 of the shell has been a little oblique, making the stria?, especially of the 

 upper valve, somewhat excentric. It wants entirely, however, the spiral form 

 of Exogyra. 



0. cretacea Morton. Syn., p. 52, pi. 19, f. 3. 



This species, described and figured from a specimen from the Eocene 

 limestone of South Carolina, is said by Dr. Morton to occur in the " older 

 cretaceous strata of Greene county, Alabama," and has been frequently 

 quoted from other cretaceous localities. I have seen but one or two valves, 

 of the so-called 0. cretacea, from the cretaceous formation, and I consider 

 them distinct. It is more oblique, and is usually somewhat curved. One 

 specimen, found by myself at Mullica Hill, is quite thick, and resembles 

 more nearly 0. robusta. The true 0. cretacea is nearly equilateral and sub- 

 ovoid. 



0. panda Morton. Syn. p. 51, pi. 3, fig. 6, exclus. pi. 19, fig. 10. 



Dr. Morton also described two species under this name. I have before me 

 full series of both. The cretaceous form, the one to which the name will 

 have to apply, is, without exception, the most irregular oyster I have ever 

 seen. It varies from crescentic to obliquely ovoid, subquadrate, oval or almost 

 circular, and is marked by from two or three undulations or contortions to 

 nearly twenty radiating ribs, which are sometimes smooth, and at others 

 coarsely imbricated. The lower valve is usually attached, but, when free, is 



[Oct. 



