NATURAL SCIENCES OF PHILADELPHIA. 163 
only in Z. Gundlachi (pl. III., fig. 10) that I have observed the 
inner side cutting point, and in this species, even, the lateral teeth 
are still sufficiently unsymmetrical 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 linguals 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. levigatus (pl. II., fig. 2), the 
first four side teeth being transition teeth. As already stated 
above, this species wants entirely the perfect laterals. In Z. cel- 
larius (pl. 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 pl. 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. inornatus, 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 Vitrinine, are 
