1 86 BuivLETiN 31 186 



p. 260. 



We have 110 hesitation in using Idea's wzxriS: alabamiensis in- 

 stead of Conrad's nasuta since, although the latter was proposed 

 first, it was already preoccupied by Sowerby's species and again, 

 Conrad's description, without figure, does not define the species. 



Plate 56 shows what appears to be the development of this 

 fecund stock from the Sabine, through the St. Maurice, Clai- 

 borne and Jackson horizons. Some of the flatter, coarsely and 

 horizontally marked earlier forms that linger along into the 

 Claiborne stage would certainly be regarded as perfectly distinct 

 species were there not so many intermediate forms connecting 

 them with the tj^pical specimens. They instinctively remind one 

 of engonata of the Vicksburg Oligocene but lack the beautiful 

 microscopic radii of that species in the Claibornian, though 

 their predecessors do show such markings. 



The left valve of the typical form is apt to be shorter than 

 the right while both show lines of growth that seem to dip or 

 plunge posteriorly (figs. 17, 19) and both show a fairly strong 

 umbonal ridge. There are sometimes traces of two grooves on the 

 post-umbonal slope (fig. 28), and in some varieties there are well- 

 marked, fine radii over considerable portions of the shell. The 

 rather small, smoother and thinner specimens with very extended 

 rostrum in the Claibornian (var. gregorioi, figs. 14, 15) are pre- 

 ceded in the St. Maurice beds by shorter, somewhat more strong- 

 ly striate forms (var. citroftella, figs. 12, 13) from the Orangeburg 

 District. The densata-\\k^& forms are apt to show a marked gen- 

 iculation as indicated in profile by fig. 28 and the post-umbonal 

 grooves are obvious and the lines of growth seem nearly horizon- 

 tal. The umbonal ridge, however is strongly and sharply de- 

 fined. 



Dall refers Conrad's 7iasuta of the Mexican Boundary Sur- 

 vey (1857, pl- i9> fig- 4) to ^ iisw species which he caXXs co7tradi. 

 What we have seen of Western Texas material inclines us to 

 believe that what Conrad really had was a somewhat elevated 

 form of alabamiensis, a mere modification of a type like the one 

 he figured on pi. 19, fig. 4 of Harris' Reprint of Fossil Shells, &c. 



Type. — Type of right valve. No. 5039 of the Phila. Acad. 



