294 



CALIFOENIA ACADEMY OF SCIENCES. 



transverse sculpture obsolete. It is thus seen that the nsperum sculj^ture is found in 

 the intermediate and apical whorls. That is, the cancellate sculpture, or, more prop- 

 erly speaking, the transverse sculpture, has been gradually forced back toward the 

 earlier stages of growth, and is wholly lost in the adult stages of B. quadrifilatum. 



The next species in the series is B. filosum. This species is found very 

 sparingly in the lower San Pedro, and although not common in the upper San Pedro, 

 is noticeably more abundant in this later horizon than in the one preceding. Its 

 separation from B. quadrijilatum no doubt began early in the Pleistocene, but this 

 type did not reach a full development until the time of the upper San Pedro. The 

 sculpture of this species consists primarily of spiral ridges or raised lines. This to the 

 naked eye seems to be the only sculpture in typical specimens, but with the aid 

 of a microscope the apical whorls are seen to have quite prominent transverse ridges, 

 giving them (with the spiral ridges) a cancellate or asperam sculpture. Thus we see 

 that the sculpture developed in the adult B. a^perum is forced out of the adult stages 

 and back into the adolescent stages, while in B.Jilosum we have the same sculpture 

 occurring only in the apical whorls, or larval stage. As the larval period is the 

 earliest in which we may study the shell of the gastropod, we may reasonably suppose 

 that in the next marked period of development this cancellate sculpture would be 

 completely lost, leaving only the typical B.Jilot^um, or spiral sculpture, to ornament 

 the whole shell from its larval to its adult stage, unless new characteristics of 

 sculpture were developed in the meantime. 



The spiral sculpture has been the persistent character in this series, while the 

 transverse has been nearly lost by being forced back further and further toward the 

 embryonic stage in succeeding individuals, until we have it remaining only in the 

 very earliest whorls of B.Jilosuni. 



It is true that all three of these species are living at the present day, and that 

 the transverse sculpture has persisted in certain individuals up to the present time, 

 but they are sufficiently differentiated to call by different specific names. It is 

 evident that in the case of the Biitluvis under discussion, the development of what 

 we call species has been brought about, not so much by the acquiring of certain 

 specific characteristics, as by the gradual loss of a certain characteristic already 

 possessed by the ancestral form. 



TABLE SHOWING DEVELOPMENT OF SCULPTURE. 



