1906.] NATURAL SCIENCES OF PHILADELPHIA. 67 



less they probably represent the curved rib stage and will be assigned 

 to it. They persist for only a short while and are followed by the tuber- 

 cled ribs of the cancellated stage. At first the svitiire tubercle is con- 

 siderably larger than the shoulder tubercle. A shoulder angle soon 

 develops, and by the end of the whorl the suture and shoulder tubercles 

 are about of the same size. 



Wliorls 3, 4 — Characterized by the cancellated stage and much as 

 in latter part of two. Changes however occur, for the ribbing above 

 the shoulder and the suture tubercles weaken, though the shoulder 

 tubercle becomes stronger. The spirals are not well marked. 



After whorl four great individual variation sets in. The cancellated 

 stage may persist through the sixth whorl. After this the individual 

 variation is still more marked. Many specimens pass through the 

 spiny stage, though it is considerably modified by the development of 

 numerous secondary spirals and riblets. In many specimens the 

 shoulder spines are long in a transverse direction and notched by several 

 spirals. Sometimes the whorl may be of a rounded shape, with the 

 spines blunt, weak and obscure. 



Reyyiarks. — Occurs in the Claiborne Fossiliferous Sand. This most 

 variable species has an apex very different from those of the other 

 forms considered in this paper. The smooth stage is nearly all con- 

 fined to the first whorl, which is extraordinarily large for Volutilithes. 

 By a careful study of its ontogeny it is plain, however, that this species 

 was derived by acceleration from the Claiborne race of V. petrosus, or 

 else from some form verj'' close to it. We are forced, then, to the con- 

 clusion that shells with rather different apices may sometimes be very 

 nearly related ; in spite of the fact that, as a rule, this condition pre- 

 cludes close affinity. Though this variable species may have given 

 rise to species and races of later age than the Claiborne, the writer has 

 no evidence to that effect. The anal siphon does not encroach up- 

 ward, and the protruded mantle smooths the preceding whorl but 

 little; nevertheless, the thickness of the shell and the irregularity of 

 growth lines in many of the older individuals point to the fact that it 

 is a senile race. It is well developed in the Claiborne horizon at several 

 points, where it is always variable and more numerous than the Clai- 

 borne race of V. petrosus. From this latter race it was probably 

 evolved during or just before Claiborne time, and may therefore be con- 

 sidered as a senile offshoot from the T^ petrosus main stock. 

 Volutilithes petrosus Conrad. 



Jackson Race. 



Locality — Jackson, ]\Iiss. 



