141 St, Maurice and Claiborne Pelecypoda 141 



The type specimen (an only one known) is from McBean 

 Creek, Burke Co., Ga., and is in the U. S. Nat. Mus. at Wash- 

 ington. D. C. 



Nut tali stock 



This common Cytheroid t3'pe of shell was well developed in- 

 to various specific forms in the Midway Eocene. But as the 

 material from that horizon is often not well preserved, the species 

 have not been well defined. 



Though the umbonal regions of the representatives may 

 be smooth and shining, there is a tendenc)^ to develop corruga- 

 tions and incised lines basall}' and anteriorly. The pallial sinus 

 is V-shaped, though often more or less blunted. 



A^uttalliopsis with several varieties occurs in the upper Sabine 

 of the Gulf states and is represented by ovata with var. Pyga &c 

 in the Virginia Basin. 



The vastly extended St. Maurice stage brings out a number 

 of nuttaliA^k& forms ; in the Claiborne the}^ reach their maxi- 

 mum in size and differentiation wdiile in the Jackson some se- 

 ciirifonnis varieties become almost indistinguishable from mittal- 

 liopsis of the Sabine. 



More specifically : While ovata in the Virginia basin gives 

 way to liciata, tents a.nd pyga, mittalliopsis seems to be followed 

 in the Gulf states by texacola and its relatives. A moderate- 

 sized shell of this species is shown on pi. 44 as fig. 12 ; and 12 a 

 shows a smaller specimen, in outline, viewed from above. Note 

 the broad development of the umbones and the extent of anterior 

 when compared with niittalliopsis. Auspicious surroundings 

 sometimes seem to have given rise to larger specimens in Texas 

 (fig.13) while less favored individuals show atavistic and special- 

 ized development (pi. 45, figs. I, 2, 3). The extreme poul- 

 soni type w^as already extant in the eastern St. Maurice stage, 

 likewise forerunners of nuttali ; but it was in the Claiborne 

 where these forms attained their most perfect development. Cor- 

 nell i seems to be a less inflated, more Dosinia-Viks. and larger 

 form of this same general stock. I have found it only at Clai- 



