68 PROCEEDINGS OF THE ACADEMY OF [Maich, 



An individual of this race was originally described by Conrad as 

 Volutalithes symmetrica}^ 



Whorls 1, 2 — Smooth and rounded. 



^V\rhQj.l 3 — ^xhe curved rib stage commences early in three 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. By the end of the whorl the shoulder 

 angle is well marked. 



Wliorls 4, 5 — Characterized by the cancellated stage. Ribbing 

 above the shoulder weakens , as does also the suture tubercle. Shoulder 

 tubercle gets stronger. Ribbing below the shoulder well marked and 

 crossed by fine spirals. 



Whorl 6 — Characterized by the cancellated stage and much as in 

 five. 



Fig. 7—Volutilithes petrostis Conrad. Jackson Race. Jackson, Miss. Apex 

 enlarged. Whorls one and two show the smooth stage. Whorl three 

 shows the curved rib stage and whorl four the cancellated stage. It will be noted 

 that the stages are all more accelerated than in text fig. 4, but less accelerated 

 than in text fig. 5. Length = 1.3 mm. 



Wliorl 7 — Characterized by the cancellated stage. Ribbing gone 

 above the shoulder, but well developed below the shoulder, and crossed 

 by fine spirals. Suture tubercles almost gone though represented by 

 a spiral. Shoulder tubercle stronger and sharper. 



Whorl 8 — Much like seven, but toward the end the ribs disappear, 

 the shoulder tubercle becomes a spine, and the cancellated stage comes 

 to a close. 



Whorl 9 — Characterized by the spiny stage. High shoulder spines 

 and no ribs. Spirals well developed, but strongest on the anterior 

 siphon. 



Wliorl 10 — In many individuals the shoulder spines tend to pass 

 into a shoulder keel, the shell thickens, and the growth lines become 

 irregular. The shelly smoothing of the preceding whorl by the pro- 

 truded mantle is only slight. This whorl is characterized by the senile 

 stage. 



" Proc. Acad. Nat. Sci. Phila., Vol. VII, p. 260. In this case "Volutalithes" 

 is probably a typographical error. 



