ZONITES. 97 



I find, however, some difficulty in deciding in all cases where the true laterals 

 end and the transition teeth commence, so gradual is the change in some 

 species. Of two Unguals of Z. intertexlus examined, I found one to have 12, 

 the other 14, perfect laterals. The number of lateral teeth in the different 

 species is given below. 



The teeth forming the gradual change from laterals to marginals are best 

 illustrated in the case of Z. leevigatus (PI. II. Fig. F), the first four side teeth 

 being transition teeth. As already stated above, this species wants entirely 

 the perfect laterals. In Z. cellarius (PI. II. Fig. G) the two transition teeth 

 have an inner lateral spur near the top of the cusp. The only lateral of this 

 species has also peculiarities in its form easily seen in the figure, but difficult 

 of description. Z. inornatus (PI. II. Fig. II) has peculiar transition teeth. 



The marginal teeth of Zonites are quite like those of Glandina and Macro- 

 cyclis (see above). The curve of the transverse rows, the rapid increase and 

 gradual decrease in size as they pass off laterally, is shown in PI. II. Figs. 

 F, G, H, and in the woodcut on p. 95. The number of marginal teeth in each 

 species examined is given below; it must be borne in mind, however, that the 

 number is not constant in any given species, though the range of variation in 

 number seems limited in the respective species. Thus, though I have found a 

 slight difference in the count of teeth in several individuals of Z. inornatus, I 

 have every reason to believe I shall never find it to have as many teeth as in 

 Z. fuliginosus. It appears, therefore, that the count of teeth has a decided 

 specific value, at least in most cases. 



The rapid increase and subsequent gradual decrease in size of the teeth as 

 they pass off laterally, though it appears usually a generic character, is some- 

 what modified in some species. Thus in one lingual membrane of Z. intertextus 

 examined, I find a much more gradual increase and decrease from the first to 

 the last marginal tooth. 



The marginal teeth in Zonites, and, indeed, all the Vitrinea, are more sepa- 

 rated than in the Helicea, and the separate rows are more widely removed the 

 one from the other, especially near the outer margin of the membrane. 



Though the simple aculeate form of marginals seems a generic character in 

 Zonites, we find the marginals bifid in Z.fulvus (PI. II. Fig. E), and bifid or 

 even trifid in Z. Gundluchi (PI. II. Fig. D), also for the first four marginals 

 in milium. This character reminds us of Vitrina (see below) ; Vitrinoconus 

 (Semper, Phil. Archip., 91); Vitrinoidea (Ibid., p. 85); Vitrinopsis (Ibid., 

 p. 86), and the numerous genera of disintegrated Nomina : also some species 

 of Limax. The first marginals of Z. exiguus have a side spur. 



Taking the general characters of dentition into consideration, Zonites is near- 

 est allied to Limax among our genera, but in the latter the marginals are gen- 

 erally more slender or spine-like, and have a less sole-like base of attachment. 



The genus Zonites being very numerous in species, it will be convenient to 

 group the species in several subgenera, founded on the form of the shell. 



VOL. IV. 7 



