1888.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 373 



may be absent or represented by a mere trace, as in Y. atyestris Aid., 

 and V. iridentata Wolf. (Figs. 4, 9.) 



Another peculiarity and really quite a prominent feature is that D in all 

 species stands a trifle more remote from the margin, as can be seen also 

 from the outside, at least in the majority of species. The position and 

 directiou of the upper fold E is so that its prolongation would reach the 

 margin just at the projecting angle or point mentioned above (Figs. 1, 

 5), and sometimes there is a slight depression corresponding to the 

 lamella. In more instances this is at the place of the lower lamella D, 

 and in some species it coincides with the lower limit of the depressed 

 part described above, corresponding to the space between the two folds. 



The accessory lamellae are in general less frequent and partly less 

 constant, aud smaller than the primaries, and thus less typical. A part 

 of them are quite constant (1) towards the margin, in the direction of 

 the coil of the shell, while others are very variable, present, or only a 

 trace, or entirely wanting; but they always occupy certain definite 

 places. These are (see corresponding numbers in Fig. 5): 



(1) On the body whorl, between lamella A and the upper (outer) angle 

 almost always nearer the margin. A lamella well developed in many 

 species, especially in V. pusilla Midler, Y. substriata Jeffr., both of 

 Europe; smaller but distinctly lamellar in V. antivertigo Drp., in V. 

 ovata Say, and in a species from Colorado (Table, So. 9) ; as a small 

 nodule scarcely recognizable as a lamella in T T . moulinsiana Drp. (Bin 

 uey's Gould) ; sometimes absent and sometimes present in Y. pentodon 

 Say, and Y. curvidens Gld., of the same shape. It certainly is the most 

 significant of the accessories and for the first-named species quite char- 

 acteristic. 



(la) In some specimens of Y. ovata Say, e. g., from Portage County, 

 Ohio, there is a small but distiuct nodule between the last named and 

 the angle of the margin ; also in occasional specimens of Y. antivertigo. 



On the body whorl, the margin is represented by an apparently more 

 or less marked callosity between the upper ends of the columella and 

 outer margin; it is really the limit of the deposit all over the body 

 whorl within the aperture. 



(2) On the body whorl, on the other side of A, between this and the 

 columella, a small nodular lamella, rather constant in Y. antivertigo and 

 Y. ovata, not infrequent in Y. pentodon and Y. curvidens Grid. 



(2a) As to P. venetzii Charp., and P. milium Gld., see text below. 



(3) Between C and D, a denticular lamella rather frequent, some- 

 times double, as in V. ovata and Y. pentodon. 



(4) Between D and E a small but quite distinct fold observed thus 

 far only in several specimens of the two last-named species and their 

 relatives. 



(5) Above E one denticular lamella, constant in F. antivertigo aud 

 7. ovata, usually present in Y. pentodon and Y. curvidens, and frequently 

 exhibited in Y. pygmcca Drap. 



