1906.] NATURAL SCIENCES OF PHILADELPHIA. 63 



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

 bling the latter part of four. 



Whorl 6 — Characterized by the cancellated stage. Ribbing above 



Fig. 4. — Volutilithes petrosus Conrad. Wood's Bluff Race. Wood's Bluff, 

 Ala. Apex enlarged. The first three whorls are characterized by the smooth 

 stage. In whorl four the passage from the curved rib stage to the cancellated 

 stage can be seen. Whorl five shows tlie cancellated stage. It will be noted 

 that the stages are all more accelerated than in text fig. 2. Length = 1.. 5 mm. 



the shoulder growing weaker, though well marked below the shoulder. 

 Suture tubercles weaker, shoulder tubercles stronger. 



Whorl 7 — Much as in latter part of six. 



Whorl 8 — Still characterized by the cancellated stage, but the suture 

 tubercles are gone, and are represented only by their spiral. Ribbing 

 above the shoulder has disappeared, though it is still present below 

 the shoulder. The ribbing below the shoulder is, however, weaker. 

 The tendency to form a shelly overgrowth is plainly seen. Though 

 its end is not w^ell marked, the cancellated stage comes to a close with 

 this whorl. 



Wliorl 9 — The spiny stage usually begins with this whorl. It is, 

 however, accompanied by well-developed senile features, the most 

 marked of wliich is the shelly overgrowth, which is produced by the 

 protrusion of the mantle on to the preceding whorl. 



Whorls 10, 11, 12 — Here we have simply an exaggeration of what 

 occurs in nine. The shelly overgrowth is extreme, obscuring the 

 ornamentation of several of the earlier whorls. The shell becomes 

 thick, the growth lines irregular, the anal siphon encroaches upward, 

 and the shoulder spines tend to pass into a shoulder keel. 



Remarks. — This is typically a senile race. No individuals of more 

 than eight whorls ever approach in their general appearance the V. 

 petrosus main stock. All specimens exhibit senile characters. These 

 latter are much more extreme than those found in the geologically 



