70 



O'V PROCEEDINGS OF THE ACADEMY OP 



distinguished from specimens taken fresh from the water. They appear to have 

 lost little, if any, of the animal matter, and preserve a perfect polish. 



The present species may be compared in form with Meretrix (Ci/therea) Mary- 

 landica Con., Miocene Foss. pi. 9, fig. 1. It differs in being more regularly 

 rounded anteriorly, beaks smaller in proportion, (the umbones being almos*t 

 acute,) and in being proportionately wider. 



Meretrix. 



? M. Yoaku mi i. Shell subquadrate; beaks prominent, placed one-third 

 of the distance from the anterior extremity, which is regularly rounded. Pos- 

 terior cardinal margin straight. Anal extremity subtruncated. Surface marked 

 by numerous very regular concentric ribs, which are abrupt on the side towards 

 the beak, and slope concavely on the other side. Crests of the ribs rounded or 

 subangular. Interspaces a little wider than the ribs. 



Length, -3 in. ; width, -4 in. ; height of valve, -09 in. 



From a brown, highly ferruginous sandstone, (Eocene), Caddo Peak, Texas. 

 Collected by Prof. Yoakum. 



The specimen being so imbedded that I could not obtain a view of the hinge, 

 renders the determination of the genus somewhat doubtful; but since it pre- 

 sents the usual appearance of this genus more strongly than of any other, I refer 

 the species provisionally as above. The shallow valve, the abrupt posterior 

 end, and the very distinct ribs (about thirty in number on the specimen before 

 me), will serve to separate the species from all the other known species. 



Protocardia Beyrich. 



There is a group, in the genus Cardium, which can always be distinguished 

 by the peculiar surface markings. This group was separated, in 1845, by Bey- 

 rich, under the above name, taking Cardium Hillanum Sow., as the type of the 

 new genus. I see no reason why it should not be considered a valid genus, 

 since it can always be distinguished by very obvious characters, and appears to 

 be fully as well founded as many ordinarily received genera. It ranges from 

 the Lias to the Eocene. I am not aware of any species described outside of 

 these limits. The markings are as follows, the anterior two-thirds to four- 

 fifths of the surface is plain or concentrically striate, while the remainder is 

 radiately ribbed. The following species belong to this genus : 



Lias. C. truncalum Phil. 



Oolite. C. semipunctatum Munst. ; C. scmiglabrum M. ; C. intextum Munst. 



Cretaceous. C. Hillanum Sow. ; ?C. puslulosum Munst. ; C. abruptum Gabb; 

 C. multistriatum Shum. ; C. Brazonsis Shum. ; C. Spillmannii Con. ; C. Colorado- 

 ense Shum. ; C. pcregrinorsum d'Orb. ; C. subhillanum Leym. ; C. impressum 

 Desh. ; C. Guerangerii d'Orb. 



Eocene. C. Nicollettii Con. ; C. diversum Con. 



?P. diversa Con. sp. A small specimen from Houston Co., Texas, from an 

 Eocene deposit presenting most of the characters of this species, but differs in 

 some few points. It has the form of the typical specimens of P. diversa except 

 that the buccal extremity is more regular, the basal margin is entire, and not 

 sinuous as in adult specimens of that species ; this may be however merely 

 the effect of the difference in age. The anterior portion is marked by obsolete 

 cancellations, and the posterior radiations are somewhat different. The ribs 

 are broad and rounded, with small bars placed at short intervals connecting 

 them, while in the young state of P. diversa the ribs are linear with wide spaces 

 and without connecting bars. In that species, also, the radiated portion of the 

 surface blends into the adjoining surface by the ribs becoming obsolete, while 

 in this specimen the same portion ends abruptly, the ribs all being of the same 

 size. It will be necessary to examine more specimens to decide whether this is 



[Nov. 



