WOOD, I'HYLOGENY OF CERTAIN CERITHIID^ 13 



velopment of this tooth is extremely variable among individuals undoubt- 

 edly of the same species; for example, among six adult individuals of 

 C. ? nodulosum showing no trace of fracture, two develop this feature 

 to a very slight extent and do not form a tooth which crosses the aperture. 



C. echinatum is referred to a different section on account of the less 

 development of this tooth, but certain individuals have the tooth devel- 

 oped to a greater extent than some specimens of C. nodulosum. 



Not only is the tooth variously developed in species which are shown 

 by their ontogeny to be related, such as C. tuherosum and C. echinatum, 

 but it is strongly developed in species undoubtedly of different descent; 

 for example, Pseudovertagu^ aluco and related species have such a tooth 

 very strongly developed, yet the wide difference between the ontogeny of 

 Pseudovertagus aluco and Cerithium tuberosum will be readily appre- 

 ciated by comparing the figures of C. tuberosum in the first column of 

 plates III to VI with those of Pseudovertagus aluco in the fourth column 

 of the same plates. 



Of the genera and subgenera of Cerithiidse already established, there 

 will be considered in this paper only the Cerithium tuberosum group and 

 others closely related to it. 



In the description of species which follow no attempt has been made 

 to give a complete synonymy. In each case the original description or a 

 reference to it is given, and also a reference to a good modern description. 

 Where necessary to make the identification of the species clearer, addi- 

 tional references are given. Descriptions are based on one individual 

 which may be considered typical of the species, but where variations such 

 as differences in color, in the number of varices and the like are cliarac- 

 teristic features, other individuals are considered. Measurements are 

 also from one individual, usually the largest of those referred to the 

 species. The method of taking measurements of the apical angle is sim- 

 ilar to that described by d'Orbigny [1842-1843, pp. 10-14], with the ex- 

 ception that in the case of convex or concave shells two measurements are 

 given representing the extreme angles obtained, and, as nearly as possi- 

 ble, the volution at which the change takes place. The sutural angle is 

 measured as described by d'Orbigny, holding the aperture of the shell 

 downward and taking the upper angle made by the suture with the right 

 side of the shell in dextral forms and the left side in sinistral fonns. 



