1906.] NATURAL SCIENCES OF PHILADELPHIA. 73 



developed spiny stage. This race of V. petrosus differs but slightly 

 from the form which we saw in the Nanafalia beds, and demonstrates 

 clearly how little a stock may vary through a long period of time if it 

 is living under favorable conditions.^- 



The phylogeny of the Alabama and Mississippi species of Volutilithcs 

 may be represented by the table on p. 74. The Yellow Bluff race is 

 provisionally placed in the Bell's Landing horizon. 



Summary. 



From the Matthew's Landing to the Hatchetigbee inclusive the 

 strata belong to the so-called Lignitic formation. They consist of 

 thick deposits of cross-bedded sands and clays, often glauconitic, and 

 of lignites alternating with thin beds of marine fossils." It is evident 

 that these deposits were formed in a great shallow arm of the sea, 

 whose w^aters were sometimes fresh or blackish and at others salt. 



The marine faunas which from time to time invaded this Lignitic 

 gulf brought with them, at first, species and races of Volutilithes with 

 normal aspect. These, however, only represent the dwellers on the 

 edge of the respective Volutilithes communities, the centres of which 

 were farther out in the open sea. These forms which were subjected 

 to the conditions of the Lignitic eventually followed out a course of 

 evolution which was a direct reflection of their unfavorable environ- 

 ment. The races of Volutilithes petrosus at Bell's Landing, Wood's 

 Bluff, and Hatchetigbee Bluff make such a series, in which the 

 senility becomes more and more extreme with the course of time." 



Occasionally throughout the later Lignitic, as at Yellow Bluff, we 

 find a race which has migrated from a more favorable environment, 

 and which resembles the primitive races of earlier Lignitic waters, as 

 those of Gregg's Landing and Nanafalia. The grade of phylogenetic 

 development which the Yellow Bluff assemblage has attained also 



'^ The author has seen forms purporting to come from the Vicksburg horizon 

 which appear to be poorly preserved examples of the Jackson race of V. petrosus. 

 He does not, however, feel justified (without further evidence) in carrying the 

 range of V. petrosus above the Jackson horizon. 



" Bull. 43, U. S. G. S., "Tertiary and Cretaceous Strata of the Tuscaloosa, 

 Tombigbee, and Alabama Rivers," Eugene A. Smith and Lawrence C. Johnson. 



^* It is to be noted in this connection that forms occur in the Miocene of Europe 

 which appear to belong to Volutilithes and to possess the shelly overgrowth. 

 The abnormal American races are produced by local conditions, that is, by those 

 of the Lignitic, and are widely separated not only geographically but geologically 

 from the European specimens. It is therefore highly improbable that the Eu- 

 ropean forms (if they are Volutilithes at all) are descended from the American ones, 

 In the absence of more definite information, we are justified in regarding the 

 phenomenon as one of parallelism. 



