1914.] NATURAL SCIENCES OF PHILADELPHIA. 575 



the canal shortening and the spire becoming more scalar during 

 individual gro^\i:h. Again the shell of F. pijrum tends to become 

 light and thin, that of F. incile thick and heavy with the progress 

 of the ontogeny. 



The sequential relations presented by F. canaliculatum and F. 

 incile are by no means apparent unless one is dealing with well- 

 preserved specimens of the latter species. The average museum 

 specimen of F. incile fails completely in this respect. If, however, 

 the material is illustrative of the entire ontogeny, the similarity in 

 the immature stages of these two species is very striking, and even 

 the late whorls and short anterior siphon of F. incile are occasionally 

 approached in very old and perhaps slightly abnormal individuals 

 of F. canaliculatum. 



When the geologic order of these species is considered in relation 

 to their morphologic sequence, we are confronted with the fact that 

 the Pleistocene and Recent (perhaps also Pliocene) F. canaliculatum 

 is morphologically intermediate between F. rugosuni of the St. Mary's 

 Miocene and F. incile of the Burwell Bay (James River) IMiocene. 

 The descent of F. incile from any known race of F. canaliculatum is 

 manifestly imi^ossible, but the derivation of F. canaliculatum from 

 F. incile could only be accomplished by the loss or non-development 

 of acquired characters and a resumption of primitive features only 

 slightly less marked than in F. rugosum. 



Again, a decision must be made on w^hat course to follow in inter- 

 preting the facts. We must either make the phylogeny agree with 

 the stratigraphic order and modify our conception of the laws of 

 evolution or else introduce a hypothetical common ancestor for 

 both F. canaliculatum and F. incile. On this basis F. incile would be 

 in no sense ancestral, but a terminal radiation of the late Miocene, 

 which left no descendants. 



In the case of the present sequence the author again favors the 

 second method of interpretation and believes that the fewer difficul- 

 ties are presented l^y the following phylogenetic scheme: 



