NATURAL SCIENCES OF PHILADELPHIA. 163 



only in Z. Gundlachi (pi. III., fig. 10) that I have observed the 

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

 are still sufficiently nnsymmetrical to be readily distinguished from 

 the centrals; in Z. Binneyanus there is also a kind of inner cut- 

 ting 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. 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. intertextus 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. Isevigatus (pi. II., fig. 2), the 

 first four side teeth being transition teeth. As already stated 

 above, this species wants entirely the perfect laterals. In Z. eel- 

 larius (pi. II., fig. 3) 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 diffi- 

 cult of description. 



The marginal teeth of Zonites are quite like those of Glandina 

 and Macrocyclis (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., fig. 1, 3, and in the several wood-cuts 

 I have given. The number of marginal teeth in each species ex- 

 amined 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 

 of Z. inomatus, I have every reason to believe I shall never find 

 it to have as many teeth as in Z.fuliginosus. It appears, there- 

 fore, 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 somewhat modified in some species. Thus in 

 one lingual membrane of Z. intertextus examined I find a much 

 more gradual and regular decrease from the first to the last mar- 

 ginal tooth. 



The marginal teeth in Zonites, and, indeed, all the Vitrininx, are 



