96 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



(PI. II. Fig. F) there is no perfect lateral, the first tooth showing a decided 

 modification or transition into the marginals. Thus we cannot say that in all 

 species of Zonites there are pure lateral teeth. It will be seen below that in 

 some species the number of laterals is reduced to two. 



I give in Fig. 18 a general view of the arrangement of the teeth in Zonites? 

 The centrals have a base of attachment longer than wide, subquadrate, with 

 lateral expansions at the corners of the lower margin. The reflected portion 

 varies in size in the various species, from highly developed in viridulus and 

 others, to slightly developed in lasmodon and others ; in the latter case re- 

 sembling the short reflection of Vitrina. The reflection always bears a more 

 or less develo, ed central cusp, generally reaching to or beyond the lower mar- 

 gin of the base of attachment, and always bearing a distinct cutting point, 

 which last, like the cusp, is usually slender, and projects over the tooth of the 

 adjoining transverse line. The side cusps of the reflected portion of the tooth 

 are usually subobsolete, but they are distinctly developed in Z. lasmodon, sup- 

 pressus, Gundlachi, placentula, gularis, arborcus, cellaring, Icevigatus, significans, 

 ferreus, viridulus, nitidus, fulvus, milium. On the side cusps are distinctly de- 

 veloped cutting points in all the species I have examined, excepting Iceuigatus 

 and cellarius, in which 1 find no trace of cutting points. These points when 

 present vary in development in the various species, generally disposed to be 

 triangular and somewhat aculeate in form, thus bearing a resemblance to the 

 cutting point of the marginal teeth. The greatest development of these cut- 

 ting points is seen in Z. capnodes (PI. II. Fig. K). The general outline of 

 the central tooth is graceful and slender as compared with the other genera, 

 except Limax and Vitrina. In most of my figures of the teeth of this as well 

 as the other genera, I have given only the size of the cutting point at its lowest 

 plane, i. e. nearest to the base of attachment. It will be understood that from 

 hence the cutting point bulges outward as it rises upwards, and again becomes 

 smaller as it arches above. At its widest development its outline is promi- 

 nent under the microscope, as in the shaded portion of the cutting point in 

 PI. II. Fio-. H, the dotted line showing at the same time the outline at its 



O ' o 



lowest plane. 



The lateral teeth in Zonites are of the same type as the central, but are ren- 

 dered asymmetrical (as usual in the land shells) by the suppression of the 

 inner, lower, lateral expansion of the base of attachment and the inner side 

 cusp and cutting point. It is only in Z. Gundlachi (PI. II. Fig. D) that I 

 have observed the inner side cutting point, and in this species, even, the lateral 

 teeth are still sufficiently asymmetrical to be readily distinguished from the 

 centrals ; in Z. Binneyanus there is also a kind of inner cutting point. As 

 mentioned above, the number of these lateral teeth varies in the respective 

 species, and is so nearly constant as to be, I believe, a good specific character. 



1 The characters of the separate teeth of this species are better shown in PI. III. 

 Fig. F. 



