1906.] NATURAL SCIENCES OF PHILADELPHIA. 61 



latter forms have a heavier shell, and the shelly smoothing of the pre- 

 ceding whorl by the protruded mantle is more marked. 



Whorl 10 — Here again we have every gradation between forms char- 

 acterized by the spiny stage, in which the preceding whorl has been 

 only slightly smoothed by the mantle, and forms which show a moder- 

 ate development of senile features. These latter show the tendency 

 for the spines to pass into a shoulder keel. They also possess a thick 

 shell with irregular growth lines, the anal siphon encroaches upward, 

 and the preceding whorl is smoothed by the shelly overgrowth of the 

 protruded mantle. Though these latter forms possess senile features, 

 these features are only slightly developed. They are, in fact, just the 

 beginnings of those characteristics which become so extreme in some 

 of the races of later time. 



Remarks. — This race occurs in the Gregg's Landing marl. The 

 greater number of individuals of the Gregg's Landing race have a 

 perfectly normal ontogeny, and in their later whorls have the spiny 

 stage well marked. These individuals belong therefore to the V. 

 petrosus main stock, and as they form a majority of the assemblage, 

 they determine the position of the race to be on the main stock. A 

 minority of individuals, however, show senile featiu'es. These senile 

 features are not well marked, but they indicate that here in this race 

 was inaugurated the tendency which resulted in the development of 

 the senile races of later time. In other words, we have here the point 

 where a senile offshoot is starting to branch from the main ancestral 

 stock. The marks of this tendency are but slight, and the individuals 

 are few; yet, nevertheless, the tendency is plainly recognizable. 

 Volutilithes petrosus Conrad. 



Bell's Landing Race. 



Locality — Bell's Landing, Alabama River, Ala. 



Whorls 1, 2, 3 — Smooth and rounded. 



Whorl 4 — The curved rib stage commences early in four and per- 

 sists for about a quarter of a whorl. It is followed by the straight, 

 tubercled ribs of the cancellated stage. The suture and shoulder 

 tubercles are about equal in size, and the shoulder angle soon becomes 

 well marked. Spirals connect the tubercles. 



Whorl 5 — Characterized by the cancellated stage and closely re- 

 sembles the latter part of foiu'. 



Whorl 6 — Characterized by the cancellated stage. Suture tubercles 

 grow w^eaker, shoulder tubercles grow stronger. Ribbing above the 

 shoulder also grows weaker, but it is well marked below the shoulder. 



Whorl 7 — Much as in six. 



