WOOD, PHYLOGE^Y OF CERTAIN CERITHIID^ 27 



latter method seems to the writer best to represent the degree of devel- 

 opment which these shells have reached. 



Cerithium dialeucum Philippi 



1851. Cerithium dialeucum Phiijppi, Abbilduugen, III, 14, pi. 1, fig. 5. 



1866. Cerithium dialeucum Reeve, Conch. Icouica, XV, No. 18. 



1887. Cerithium dialeucum Tryox, Manual of Conch., IX, 130, pi. 23, figs. 

 87, 88. 



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

 nitz, Bd. I, Abth. 26, 167, pi. 31, figs. 8, 9. 



Measurements : Length, 31.4 mm. ; greatest diameter, 1.5 mm. ; apical angle, 

 45°, changing to 25.2° on the last two volutions; sutural angle, 79°. 



Color : Grayish white on the spirals with alternating bands of dark reddish 

 or purplish brown in the grooves between the spirals. 



The youngest volution preserved is 1.5 mm, in diameter, but it reveals 

 nothing of the development at this stage, for the ornamentation is nearly 

 obliterated on the first three volutions. The third volution, however, 

 indicates the presence of two strong spirals crossed by ribs. The fourth 

 volution has an oblique-angled outline formed by two primary spirals 

 and three intercalated ones. Spirals of the first, second and third orders 

 are present on the shoulder and all are crossed by prominent ribs, with 

 varices to the number of about three on a volution. The different orders 

 of spirals on this shell are clearly indicated by a marked difference in 

 size. The two primary spirals remain of equal strength for about six 

 volutions, after which the lower becomes somewhat weaker and one of 

 the spirals on the shoulder becomes stronger, so that on the last two volu- 

 tions the shell has three strong spirals, of which the median one is the 

 most prominent, and intercalated spirals to the fourth order are also 

 present. The whorls of the neanic shell are rounded in outline, and only 

 the later portion of the body volution becomes somewhat angular. The 

 lower slope of the body whorl bears six or more strong spirals with inter- 

 calated ones, all of which tend to become nodose toward the end of the 

 whorl. 



The aperture is oval. The callus of the inner lip is narrow, with a 

 prominent posterior tooth. The anterior canal is oblique and of moder- 

 ate length. The interior of the outer lip is deeply grooved to correspond 

 with the spirals of the outer surface. The grooves of this surface are 

 white like the spirals, and the ridges are colored to correspond with the 

 depressions of the outer surface. A narrow margin of the outer lip is 

 thick, smooth and white. 



Horizon and ix)caijty : Recent. Philippines. 

 No. 20132. Columbia University collection. 



