''^'^^ PROCEEDINGS OF THE ACADEMY OF [May, 



whorls to the latest, and the changes in shell features and ornamenta- 

 tion noted in the individual have been correlated with characters 

 observed in the species of past geologic time. Where phylogenetic trees 

 have been constructed they have been based on the principle that the 

 mdividual in its development repeats, in its early stages, shell features 

 which are found in the adult individuals of its ancestors. The authors 

 have, for the most part, followed the principles applied and the laws 

 formulated by Hyatt in his classic studies of the Cephalopoda. 



In any phylogenetic study of the gastropod shell, acceleration of 

 the characters plays an important part. With the exception of Fusus 

 and Voluta, most of the genera studied have been rather highly spe- 

 cialized and accelerated types like Fulgur and Strotnbus. In such forms 

 many of the stages have been hurried back into the earlv whorls 

 slurred over or lost. " ' 



The writer feels that a detailed study of the shell features in some 

 species of Volutilithes, as Volutilithes petrosus of the American Eocene 

 may add to our knowledge of the normal order in which such features 

 are evolved. Several species of Volutilithes go through quite a number of 

 stages of development in ornamentation, and there is little acceleration. 

 Each stage, of course, does not necessarily represent an adult ancestor, 

 for at present it is not known how much unequal acceleration of 

 . ancestral characters has come into play, yet nevertheless the writer 

 feels that here we have a fairly normal and even succession of develop- 

 mental features. 



The occurrence of certain characteristics with considerable con- 

 stancy m the later whorls of many individuals of Volutilithes petrosus 

 has led the author to consider such features in other genera of gastro- 

 pods. These features will be treated in the sections of this paper 

 entitled Senility and Senile Offshoots from Main Ancestral Stems. 

 The results of this later investigation are applied to Fulgur carica, 

 in an endeavor to unravel its ancestry. 



Normal Progressive Development. 

 In recording the characters which occur at different periods in the 

 growth of any gastropod shell it is necessary to be exact; and this 

 frequently results in the use of terms which appear arbitrary. The 

 writer has, throughout this paper, endeavored as far as possible to 

 apply the terms previously in use. Markings on the outside of the 

 whorl w^hich run parallel or nearly parallel to the suture are described 

 as spiral; while those running across these spirals are described as 

 transverse. The transverse markings which first appear are referred 



