WOOD, PHYLOGENY OF CERTAIN CERITHIID^ 3 



and order of introduction of the various features of the shell are indica- 

 tions of relationship, and the comparison of a sufficient number of life 

 histories will furnish a pliylogenetic tree whose completeness depends 

 upon the abundance and perfection of the material available for study. 

 In making such comparisons, it is important to take account of paral- 

 lelism in development, in consequence of which similar characteristics 

 may appear for a greater or less portion of the life history in individuals 

 belonging to divergent groups. An illustration of this is to be found in 

 VicinoccriUiium houei Desii. and V. parallclum sp. nov. of the Eocenic 

 of the Paris basin. These shells are closely similar in the adult and have 

 been referred to the same species, but they differ in developmental his- 

 tory. Both have, in the adult, one extremely prominent spiral around 

 the middle of the whorl, with strong ribs crossing it and a less prominent 

 spiral below. The remainder of the surface is covered with spirals of 

 secondary, tertiary, and higher orders. In the young, Yicinocerithium 

 houei has three spirals, the lowest of which is the most prominent (])late 

 VII, fig. 6). Later, in the growth of the shell, finer spirals are inter- 

 calated, and all are crossed by ribs. At a still later stage, the upper 

 spiral of the three primary ones becomes the most prominent and finally 

 develops into the strong carina of the adult (plate viii, fig. 6, and plat;.' 

 IX. figs. 5, 6). Vicinocerithium parallelum also begins with three spirals, 

 of which the lowest one is the most prominent, but ribs are present as 

 soon as the third spiral appears (plate vii, figs. 4, 5). Later, the median 

 spiral becomes as prominent as the lower, and for several volutions the 

 two are equally prominent, while the upper spiral diminishes in propor- 

 tion, and additional fine spirals are introdiiced on the shoulder thus 

 formed (plate viii, figs. 4, 5). The median spiral continues to increase 

 until it forms the carina of the adult (plate ix, figs. 3, 4), a feature 

 which was formed in the preceding species by increase in the upper spiral 

 of the three primary ones. These shells are parallel in the adult, but 

 differ in development and are to be traced to different ancestors. 



The phylogenetic record is obscured by the fact that not all of the 

 history of a group is expressed in the ontogeny of a single species. As 

 new characters are introduced in the evolution of the group the record 

 becomes too long to be repeated during the lifetime of a single individ- 

 ual, and each of the ancestral stages occupies a shorter and shorter por- 

 tion of the length of the shell, until some stages disappear altogether. 

 An individual thus shows the adult characteristics of its ancestors at an 

 early period of its life history. These facts are expressed as follows in 

 Hyatt's law of acceleration :^ 



» A. Hyatt : "Genesis of the Arietidse." p. ix. 1889. 



