WOOD, PHYLOGENY OF CERTAIN CERITHIID^ ig 



growth does not, however, form a distinct tooth as in Cerithium tuber- 

 osum and some forms of C. echinatum. 



Horizon and localities : Recent. Senegal, west coast of Africa and spar- 

 ingly near the mouth of the Gambia River. M. Cossmann [1906, p. 66] as- 

 sumes that this shell is a fresh-water form, but this is hardly borne out by 

 the statement of Adanson, who collected the animals in their native locality. 

 According to his account of the habitat quoted above, it seems that the animal 

 is typically marine, and comparatively few individuals have migrated up the 

 Gambia River. 



No. 20125, Columbia University collection. 



Museum of Comparative Zoology, museum collection. 



Remarks : This shell, from the protoconch to the seventh volution, 

 corresponds exactly in development with Cerithium tuberosum. It is, 

 however, a more accelerated shell, since the stage with two equal spirals 

 persists in C. tuberosum for ten volutions, while in C. adansoni this 

 feature is lost on the sixth volution. The adnlt of C. tuberosum cor- 

 responds approximately to the seventh volution of C. adansoni, but the 

 correspondence is not exact, for at this stage 0. adansoni has not ac- 

 quired a sub-sutural row of fine nodes, while its ribs have become even 

 more spinose than those of C. tuberosum. C. adansoni is not to be re- 

 garded as a descendant of C. tuberosum, but rather the two are descended 

 from a common ancestor, C. adansoni passing through its ancestral stages 

 rapidly and adding new characters, while C. tuberosum is retarded in its 

 development and never attains the high degree of ornamentation charac- 

 teristic of the adult C. adansoni. 



Cerithium echinatum Lamarck 



1843. Cerithium echinatum Lamarck, Animaux sans vert, M. II, IX, 291. 

 1887. Cerithium echinatum Tryon, Manual of Conch., IX, 12.3, pi. 20. figs. 25-27. 

 1898. Cerithium echinatum Kobelt, Syst. Conch. -Cabinet von Martini u. Chem- 

 nitz, Bd. I, Abth. 26, 100, pi. 20, figs. 5-8. 

 1906. Oonrmyia echinatum Cossmann, Essais de Pal6oconch., VII, 69. 



Measurements : Length, 49.4 mm. ; greatest diameter, 27.3 mm. ; apical 

 angle to the ninth volution. 45°, changing to 37.5° ; sutural angle, 82.5°. 



Color : Grayish white, sometimes marked with transverse patches of brown, 

 which is deepest on the spirals. 



The protoconch of Cerithium echinatum is much worn on the only 

 available specimen showing that feature, but so far as can be determined, 

 it is precisely like that of C. adansoni, and the first four volutions are 

 indistinguishable on the two shells. On the fifth volution there is less 

 contrast between the primary spirals and those of higher order than on 



