WOOD, PHYLOGENY OF CERTAIN CERITHIID^ 5 



this type of outline at about the seventh volution. C. graciliforme is 

 therefore a more retarded shell than C. echinatum in this respect. In 

 the acquisition of nodes, however, it is accelerated, for it acquires nodes 

 on the ninth volution, while C. echinatum does not develop them until 

 the thirteenth volution. This differential acceleration and retardation 

 often produces a wide difference in the appearance of adult shells, with- 

 out the introduction of any new character. 



Individuals having the same development for a greater or less portion 

 of their life history, but differing in the adolescent or adult stages, may 

 be regarded as divergent descendants from the same ancestral stock. A 

 striking illustration of such divergence is found among recent species of 

 Cerithium in the group of which C. tuberosum may be taken as a type. 



In addition to the types of variation already noted, there is a kind of 

 individual variation which seems to differ from all, and that is in the 

 slight accentuation of the characters of the shell from their earliest ap- 

 pearance to the last volution. This is seen in individuals of Cerithium 

 lamellosum, some of which show secondary spirals distinctly on all the 

 whorls, while on others these spirals are but faintly indicated. This may 

 be due to some condition in the environment, possibly to more lime in 

 the water or to better food supply in the case of the well-marked indi- 

 viduals, or it may be due only to an inherent tendency to variation. 



Acceleration and retardation and the introduction of new characters 

 may cause divergence sufficient to serve as a basis for the separation of 

 species, or they may appear to a less degree in individuals referred to the 

 same species. Where many individuals are present, we find gradations 

 in the various characters compelling us to establish varieties, and the 

 more extensive the material the more insensible the gradations become, 

 so that if our collections were sufficiently extensive, it would doubtless be 

 possible to establish a perfect gradational series among various species of 

 Cerithium, as in the classic example of the Planorhis of Steinheim.^ 



As might be expected, the greatest difficulty encountered in determin- 

 ing the phylogeny of Cerithium has been due to the scarcity of material. 

 It has been impossible to secure specimens of shells from the early Meso- 

 zoic horizons which might be expected to furnish the ancestors of the 

 forms occurring in such alnmdance in the Eocenic. A similar difficulty 

 arises in connection with late Tertiary and early Quaternary material in 

 which we should expect to find the connecting links between Eocenic and 

 recent species. In the absence of specimens of shells, figures and de- 

 scriptions have been freely used, but the figures of the early portions of 



■'A. Hyatt: Mem. Boston Soc. Nat. Hist. 1880. 



