Occasional Papers of the Museum of Zoology 53 



tions (example G. calverti Pilsbry) will be found to belong to 

 any of these groups in its strict sense. 



in the radulae examined of these three groups, Guppya, 

 Habroconus, and Euconulus, three more or less distinct ten- 

 dencies or trends seem to be present. 



1. A tendency for all of the teeth to become elongate and 

 for the outer teeth to turn inward and to lose the ectones. The 

 marginals of all three groups have lost the ectones, but the 

 central and laterals of Guppya s. s. have not been affected to 

 any great extent. Both the centrals and the laterals of the 

 other two groups are elongated, although in Habroconus this 

 is accomplished by the increase in size of the distal portion of 

 the reflected edge, while in Euconulus the cusps themselves 

 have been lengthened to a greater degree. The laterals of 

 both of the last two groups show a progressive tendency, from 

 the center out, for the ectocones to move up on the outside of 

 the mesocones and finally to diminish in size. 



2. A tendency for the ectocones to be reduced in numbers. 

 This has not affected Guppya s. s. as much as the others, as all 

 of the marginals have at least two ectocones. Half of the well- 

 developed marginals of Habroconus still retain two ectocones, 

 while none of the large marginals of Euconulus have more 

 than one. The bicuspid teeth tend to move back into a less 

 oblique position than that of the tricuspid. This tendency 

 towards reduction of the number of cusps on the marginals 

 is carried still further in such genera as Zonitoides, where the 

 definitive marginals are mostly unicuspid. 



3. A separate tendency, at least somewhat coincident with 

 size, to reduce the number of teeth in the transverse rows. 

 Thus, the largest species, G. trochulina, has about 1 19, E. fnl- 

 vus about yj, B. elegantulus about 85, G. gundlachl 65, and G. 

 stcrkii 41. Up to a certain point, fliis appears to go on more 



