52 BULT.ETIN 31 53 



shell thill ; beaks small, pointed ; ribs about thirty-three, obsoletely tuber- 

 culated on the anterior portion ; teeth lamellar, oblique ; cicatrices scarcely 

 perceptible ; cavity of the shell rather shallow, margin crenulate. 



Diam. . . lyength .2, Breadth 7-2oths of an inch. 



This seems to be a rather characteristic species of the mid- 

 Eocene of the more central Gulf States. As is well known it 

 was first figured by Lea from a very young specimen, yet entire- 

 ly recognizable. Dall has straightened out the erroneous refer- 

 ences of this species to cuculloides and the excellent illustrations 

 of Sheldon have given the final touches in the matter. 



This species shows considerable variation in accordance with 

 horizon and locality. A few of the more common forms may 

 here be mentioned. 



At Claiborne (sand-bed) the average adult is about 20 mm 

 in length, though some specimens may reach 35 mm. In the 

 specimens from the base of the bluff and at Lisbon, 30-35 mm is 

 a common limit ; but in the Orangeburg District, S. C. some spec- 

 imens reach 45 mm . Small and rare specimens only are known 

 from Texas (see var. parsaba along the Sabine). 



Claiborne sand specimens vary among themselves consider- 

 ably so that, while in the main Lea's description holds true, there 

 is apt to be a rather noticeable gibbosity in the centro-anterior 

 portion of the shell. The medial portion of the shell is more fine- 

 ly ribbed than the extremities. This comes about somewhat by 

 bifurcation of the ribs but mainly by implantation The anter- 

 ior ribs are generally more or less grooved, likewise the sub-lig- 

 amental ones. The heavy ribs on the umbonal ridge are the last 

 to be subdivided or even channelled, practicallj^ never so in "sand 

 bed" shells. The number of primal ribs is in the neighborhood 

 of thirt}^ In the St. Maurice beds below (at base of Claiborne 

 Bluff and at Lisbon, var. subscopula, n. var.) the primal ribbing 

 shows a greater number, about 40, the subdivision (by bifurca- 

 tion) is quite evenly carried out over the surface of the shell. 

 The posterior basal margin is apt to be more acute, the anterio- 

 basal margin more obtuse. The Orangeburg District specimens 

 are again practically of the Claiborne type, though larger and 



