WOOD, PHYLOGENY OF CERTAIN CERITHIIDJE 51 



whorls, and the form of the spirals changes to the imbricated type de 

 scribed above. On account of this divergence expressed in the adult 

 shell, C. lamellosum may be considered a lateral branch from the line 

 developing in the direction of C. tuberosum. No descendants of C. 

 lamellosum have been found in the material studied, and it may have 

 died out at the end of the Eocenic without giving rise to later species. 



The young of C. lamellosum gives a clue to the kind of development 

 which preceded the stage with ribs and two strong spirals, and it is to be 

 expected that from such primitive conditions development would take 

 place in several different directions. This we find to be the case, as illus- 

 trated by several of the following species. 



M. Cossmann refers this species to Ptychocerithium on account of the 

 narrow opening of the canal and the strong varix opposite the outer lip. 

 He considers it as closely related to Yulgocerithium , but from the devel- 

 opment of the early stages it seems to be more closely related to Cerithium 

 than to either of these. 



Cerithium inabsolutum Deshaycs ? 



1866. Cerithium, inabsolutum Deshayes, Desc. des aniiuanx sans vert, decou- 



verts dans le bassin de Paris, III, 170, pi. 74, fig. 28. 

 1906. Ptychocerithium inahsolutum Cossmann, p. 80. 



Measurements (young individual): Lengtli, 10 mm.; greatest diameter, 4 

 mm. ; apical augle, 23.5° ; sutural angle, 8.5°. 



Two very small individuals probably represent the young of C. inabso- 

 lutum, but are referred to that species with doubt, since no adult is avail- 

 able for comparison with them. 



A part of the protoconch is present on one specimen and, so far as can 

 be determined, is like that of C. adansoni. The first complete volution 

 is .3 mm. in diameter and bears two continuous spirals, one of which 

 appears slightly before the other. This type of ornamentation continues 

 for six volutions, but on the seventh ribs are developed, fine spirals appear 

 on the shoulder and a spiral is intercalated between the two primary 

 spirals. Strong varices to the number of one or two to the volution are 

 developed on the remaining whorls. On the eighth volution a constric- 

 tion below the suture defines a sub-sutural band which is raised into 

 nodes where crossed by the ribs, and numerous spirals of higher order 

 are introduced. The shell comprises eleven volutions, and the two 

 strong, equal spirals characteristic of Cerithium remain undiminished in 

 strength on the last volution present. The lower slope of the body volu- 

 tion bears two strong spirals with three finer ones below them. 



