5" 



THE AMERICAN NATURALIST. [Vol. XXXVII. 



one with five volutions), while the shoulder angle at the same 

 time retains its tubercles. In more advanced specimens the flat- 

 tening, which begins earlier, is bordered by a carina arising in 

 the third or fourth whorl of the conch, while the shoulder 

 becomes excavated below this. The tubercles of the shoulder 

 angle cease about a whorl later and a simple carina succeeds. 

 In a still more accelerated specimen the flattening next to the 

 suture is depressed on the formation of the bounding carina on 

 the third whorl, while acceleration is also shown by the presence 

 of intercalated spirals on the shoulder of that whorl. In the 

 accelerated types the depressed sutural canal becomes triangular 

 from the development of the 

 bounding carina into a strong 

 posterior notch. This is empha- 

 sized, as acceleration increases, 

 by the disappearance of the 

 shoulder angle ; which gives us, 

 when the spire is low, the type 

 named by Conrad Busicoti exca- 

 vates (Fig. 5) but a type very 

 fig. 5 . sycoty/ms different in appearance when 

 After'Sraa X 4 ' high. (See S. elongatus Gill, 



150). Finally in these beds (Magnolia, Dupli 

 sonian collection, 1 14540) occur specimens showir 

 tion between .V. cxcavatus and the extremely ac 

 described by Tuomey and Holmes as CassiduL 

 (Fig. 6) from the Pliocene (?) of South Carolina 

 vatus" type of aperture is, however, not confinec 

 descent, but crops out in parallel lines, i. e is 

 character. This is shown hv nlrl ™ _i 



