THE AMERICAN NATURALIST. [Vol. XXXVII. 



quite obsolete in the more accelerated individuals, thus giving the 

 lip a rounded "excavatum" form. Here the notch made by the 

 sutural ridge is the strongest element, having entirely replaced 

 the original notch of the outer or keel portion. This is com- 

 monly accompanied by a looser coiling, because the succeeding 

 whorls clasp below the middle of the preceding one instead of 



Miocene Yorktown beds, and terminated in a high-spired and 



shoulder angle. ( Fig. 7b.) A form of this kind is not unlike in 

 appearance to that of an old Buccinum in whic h the last whorl 

 has been partially separated. The spirals of the shoulder are 



shoot from a primitive line with few spirals, or a partial retarda- 

 tion affecting this feature. This small number of spirals, in 5. 

 incilis seems to be due to the rapid widening of the sutural chan- 

 nel and the consequent encroachment of the bounding ridge on 

 the shoulder space. The latter remains always narrow, thus 

 preventing a material increase in the number of spirals. 



Apparently in the direct line of ancestry of S. incilis is .S'. 



alvcatus (Conrad) (Fig. 8). This preserves 

 the keel throughout and has the strong ridge 

 bordering the sutural channel. With our 

 present knowledge we may perhaps regard .V. 

 alvcatus as the radicle of this line, which 

 seems to have branched off from the main 

 line in Miocene time. The changes are 

 towards a high spire and a profound sutural 

 canal, terminated by an excavatum-like lip. 

 .V. incilis may therefore be considered as 

 L < 3 ^SLcao. representing a phylogerontic branch in mid- 

 Miocene time which was not propagated 

 beyond that period. 



In the Duplin beds occurs a high-spired form described by 



