72 PROCEEDINGS OF THE ACADEMY OF [Mai'Ch, 



In the Nanafalia the first member of the Volutilithes petrosus assem- 

 blage makes its appearance, and is representative of the V. petrosus 

 main stock. After the Nanafalia the races of V. petrosus are a promi- 

 nent feature of the faunas. Though the adults of these races may differ 

 greatly, the young resemble each other closely, and indicate their 

 common derivation from V. limopsis. 



In the Gregg's Landing marl, the Gregg's Landing race represents 

 the V. petrosus main stock, though a few of its individuals show slightly 

 senile tendencies. In the Bell's Landing marl the Bell's Landing race 

 has separated from the main stock with the evolution of senile features. 

 It is the first member of a senile offshoot which in later time produces 

 the Wood's Bluff and Hatchetigbee Bluff races. These latter are 

 evolved from the Bell's Landing race, not so much by an acceleration 

 as by an exaggeration of the marks of senility. The Hatchetigbee 

 Bluff race is without descendants and the last term of this senile series. 

 Its individuals are dwarfed and stunted, reflecting plainly the unfavor- 

 able conditions under which they lived, and which soon caused the 

 extinction of the race. 



While in some localities the members of the Volutilithes petrosus 

 assemblage were following out a line of evolution which resulted in 

 their elimination from the geological record, the main stock was still 

 flourishing in nearby regions. For instance, at Yellow Bluff on the 

 Alabama river we have a representative of the V. petrosus main stock. 

 The YeUow Bluff race is indicative of favorable conditions at that 

 locality. 



In the Lower Claiborne the V. petrosus main stock occurs, though it 

 is represented only by a few poorly preserved specimens. The Clai- 

 borne, however, not only gives us the Claiborne race but an offshoot 

 from it, namely, V. sayana. This Claiborne race is an assemblage of 

 rather accelerated and variable forms which probably express the 

 effect of shallow water marine conditions upon the main stock. By 

 an acceleration of ornamental features V. sayana was derived either 

 from the Claiborne race or from some very closely related form. 



Passing now into the Jackson, we find the last member of this phy- 

 logenetic series which we have traced up from the Matthew's Landing 

 horizon. It is the Jackson race of V. petrosus. This form is the result 

 of the slow and even evolution of the ancestral race which took place 

 in the favorable conditions of a more open sea. Its ontogeny is nor- 

 mal, each stage following its predecessor in regular fashion. Though 

 this form is accelerated, there is little mingling of the senile and spiny 

 characters by unequal acceleration. The senile stage closes a well- 



