40 ANNALS NEW YORK ACADEMY OF SCIENCES 



On the fourth of the volutions preserved one spiral is intercalated between 

 each pair of primaries and the upper primary spiral becomes weaker, 

 making the shoulder more pronounced. The sixth volution has three 

 fine spirals intercalated between the two strong primary ones, with one 

 above and one below the third primary spiral. The lower slope of the 

 volution has at this stage two spirals, one stronger than the other. The 

 same type of ornamentation persists throughout the growth of the shell, 

 with an increase in the number of spirals, which vary in strength accord- 

 ing to the order of their introduction. The third primary spiral on the 

 shoulder remains always stronger than any secondary spiral, Ijut not so 

 strong as the other two primaries. The latter become very prominent 

 and produce the well-defined oblique-angled outline of the volution so 

 characteristic of Ceritkium tuberosum. 



The aperture has the usual oval form of the shells of this group, with 

 a narrow, thick callus and prominent posterior tooth. The anterior canal 

 is short and widely open. 



Horizon and locality : Pliocenic of the Caloosahatchie beds, Florida. 

 No. 12569, American Museum colleetiou. 



Remarks : The first three volutions preserved on this shell recall the 

 seventh, eighth and ninth volutions of the Eocenic species, Ccrithiuni 

 retardatum (plate iv, fig. 10; plate v, fig. 9). On the latter shell three 

 spirals are developed, the two lower of which are stronger than the upper, 

 and the first intercalated spiral appears between the strong spirals. A 

 similar development is seen on C. callisoma, but it is more accelerated 

 than C. retardatum, since its shoulder is distinguishable from the first, 

 while C retardatum never acquires a distinct shoulder, but retains the 

 primitive rounded outline of its volutions throughout life. 



C. callisoma is most closely related to the recent species C. tuberosum, 

 but it is more primitive than the latter species, since it retains tlie oblique- 

 angled outline of the volutions throughout life without a trace of weaken- 

 ing of the lower spiral, while on C. tuberosum the lower primary spiral 

 becomes gradually less prominent, imtil on the body volution the outline 

 is sharply angular. The resemblance of the adult C. callisoma to the 

 young C. tuberosum is most striking, and if the latter species were from 

 a less distant locality, we might at once assume it to be the Pliocenic an- 

 cestor of C. tuberosum. As it is, we may suppose that C. callisoma is 

 the American representative of this ancestor, which existed in the vicinity 

 of the Eed Sea or the Indian Ocean, but has not yet been recorded from 

 that locality. 



