56 ANNALS NEW YORK ACADEMY OF SCIENCES 



Cerithium sp. undt. 



Measurements : Length, 13.5 mm. ; greatest diameter, 5.3 mm. ; apical angle, 

 35°, changing to 26° on the last three volutions; sutural angle, 83.2°. 



Two small specimens of nine volutions each are the only representa- 

 tives of this species in the collections studied. They are probably young 

 individuals, but it is impossible to identify young shells from the figures 

 and descriptions as usually written. 



The youngest volution preserved is .4 mm. in diameter and has one 

 spiral only. The next volution is too much worn to show the ornamenta- 

 tion, but the third volution has two spirals crossed by ribs. The two 

 spirals become of equal strength and, with the ribs, remain the only orna- 

 mentation until the sixth volution, when a fine spiral is introduced below 

 the lower spiral. On the next volution a spiral is intercalated between 

 the two primary ones, and on the eighth volution a fine spiral appears on 

 the shoulder. The body volution retains the characteristic two strong 

 spirals and has two fine spirals on the shoulder, with one intercalated be- 

 tween and one below the two primary spirals. The lower slope of the 

 body volution has two strong spirals and several finer ones below the 

 primary spirals. Spirals of the third order also appear on this lower 

 slope. 



The aperture is circular, with a shallow anterior canal. The callus of 

 the inner lip is comparatively wide and has a distinct ridge near its pos- 

 terior end. The outer lip is thin and not crenulated on the young shell. 



Horizon and locality : Eocenic, Paris Basin. Precise horizon and locality 

 unknown. 



No. 20163, Columbia University collection. 



Eemaeks : This little shell is like C. hicarinatum to the sixth volution, 

 after which it develops in a direction different from that of the two de- 

 scendants of C. hicarinatum already described, namely, C. hicarinatum 

 mut. trispirale and C. retardatum. This divergence is expressed in the 

 acquisition of a third spiral on the lower slope of the whorl, instead of 

 on the shoulder, as in the line of evolution just described. The spirals 

 of the shoulder slope are introduced later, but always remain fine, and 

 the shoulder persists to the last volution present on these forms, while it 

 is lost on C. hicarinatum mut. trispirale. This species may be consid- 

 ered a descendant of 0. hicarinatum. but it represents a different path of 

 evolution from that, including C. retardatum. 



