1922] NATURAL SCIENCES OF PHILADELPHIA. 33 



of Troschel (1856-63) that they are without cusps. In every 

 species that I have examined under an oil-immersion lens, they 

 could be detected. 



Two lines of development appear in the structure of the marginals. 

 The type that is here considered the most primitive is that repre- 

 sented by Hendersonia. In this and related groups (Table I), there 

 are a comparatively small number of teeth .with one or two major 

 cusps and many minor ones arranged along the outside of the tip. 

 In Helicina there is an evident tendency for the cusps to arrange 

 themselves on the extreme tip of the tooth, while many West 

 Indian genera show a tendency to accentuate the one or two major 

 cusps. The climax of the first line of development is seen in H. 

 zephyrina, where the cusps of each tooth form a nearly transverse 

 row, with the largest teeth in the center of the terminal disc; 

 each tooth is so twisted as to bring the transverse crown of cusps 

 in a line nearly at right angles to the long axis of the radula. The 

 other line of development finally results in the unicuspid inner 

 marginals of Sioastoma and the Vianinae (Trochatella). 



On account of their complexity, and the vertical position of the 

 numerous marginals, the radulae of the Helicinidae are peculiarly 

 difficult to study. For this reason it is often necessary to examine 

 quite a few specimens and to view the teeth in every possible position, 

 in order to correctly interpret their shape. The best results were 

 obtained by cutting each radula transversely into several pieces 

 with a rather dull-edged scalpel, so as to leave torn edges. Each 

 piece must be carefully arranged and then the cover-glass dropped 

 on suddenly to flatten the ribbon. In order to study the accessory 

 plate it is absolutely necessary to get a place where the marginals 

 are missing or turned back. Separated capituliform complexes 

 are also of great aid, although the accessory plate, and especially the 

 wing, is only too often broken in the process. If the centrals are sepa- 

 rated from the laterals and marginals, the cover-glass usually flattens 

 them into a position where it is very difficult to study them. In 

 all cases, except that of Helicina neritella, the centrals, laterals, 

 and the tips of the marginals, are shown in the figures as nearly 

 as possible to the natural position, so as to make the views uniform. 



When one becomes accustomed to the changes in apparent 

 shape, due to different view points, the radulae of different specimens 

 of the same species show remarkably little fluctuating variation. 

 The centrals and laterals rarely show a variation of more than a 



