1906.] NATURAL SCIENCES OF PHILADELPHIA. 75 



indicates that a normal, slow, and even evolution has been going on 

 in one locality ; while at the same time rapid, senile evolution has taken 

 place among forms subjected to unfavorable conditions in another 

 region. 



Where, then, was the centre of this normal phylogeny? We may 

 well look toward the open sea to find this favorable environment, and 

 the deposits which follow the Hatchetigbee beds furnish us with the 

 desired answer. 



With the inauguration of the Lower Claiborne began that movement 

 which finally resulted in the deepening of the sea and the production 

 of the white limestone of the Jackson. The brackish water conditions 

 disappear and are replaced by those of a shallow water (but typically 

 marine) environment. The Claiborne race of Volutilithes petrosus 

 shows the effect of such conditions upon the main stock. The sinking 

 of the sea-bottom proceeded throughout Claiborne time, and finally 

 the Jackson limestone with its relatively deep water fauna is intro- 

 duced. The Jackson race of V. petrosus represents the result upon the 

 main stock of slow and even evolution in a favorable environment. 



It follows, then, that every stock has some particular set of condi- 

 tions in which it develops normally. Contemporaneously with this 

 normal phylogeny a senile evolution in the same stock may occur at 

 some less favored locality. In the forms we have considered the 

 environment most favorable to a slow and even evolution is an open 

 sea one, in which limy deposits are forming. It is only, then, in a 

 thick limestone deposit that we could expect to find all the phyloge- 

 netic stages of the Volutilithes petrosus main stock and other primitive 

 species. In conclusion it is interesting to note that one of the only 

 two existing species of the genus, namely, Volutilithes abyssicola 

 Adams and Reeve,^^ is a cancellated form which apparently is quite 

 close in its grade of evolutional development to the ancestral V. limop- 

 sis of remote Eocene time. This recent species is found in relatively 

 deep water off the Cape of Good Hope, and probably is a member of 

 a primitive stock, which iri the favorable environment of a deep and 

 open sea has been able to persist long after its nearest relatives have 

 become extinct.^^ 



The writer wishes to express his indebtedness to Prof. Henry A. 

 Pilsbry, of the Academy of Natural Sciences of iPhiladelphia, and to 

 Prof. Amos P. Brown, of the University of Penns3dvania, for many 



^^ Zoology of the Voyage of the Samarang. 



i« See Dall, Trans. Wag. Inst., Vol. Ill, p. 74. 



