NATURAL SCIENCES OF PHILADELPHIA. 227 



The base of attachment is, however, unsymmetrical by the suppres- 

 sion of both upper and lower inner lateral expansion ; the upper 

 margin is simply squarely truncated. Above the centre of the base 

 of attachment springs from its surface the gouge-shaped, rounded, 

 gradually expanding cusp reaching nearly the lower margin of the 

 base of attachment and produced into a still more expanded, 

 bluntlj^ truncated cutting edge (one cannot call it a cutting point), 

 which projects far beyond the lower margin of the base of attach- 

 ment on to the teeth of the next transverse row, and is also greatly 

 expanded on the outer side, so as to overlap the adjoining tooth. 

 This cutting edge is slightly incurved at its centre. There is one 

 point of difference between the central and adjoining marginal 

 teeth which is verj?^ mai'ked ; in the centrals the lower margin of 

 the base of attachment is more expanded than the cutting edge, 

 the reverse of which is found in the marginals. 



The marginals retain this general form to the extreme edge of 

 the membrane, but they decrease greatly in size upon the edge. 

 The outer marginals have to their cusps a small side spur, gouge- 

 shaped as the cusp itself; the extreme marginals have such a spur 

 at either side. In both cases the cutting edge springs from the 

 outer side of this side spur, which must be considered as repre- 

 senting the side cusps of the usual Helicinse type of dentition. I 

 have elsewhere (Ann. Ljx. N. H. of N. Y., XI., 39) shown that 

 this type of tooth is but a modification of the usual type brought 

 about by the expansion, bluntly rounding and shortening of the 

 cusps, and the still greater expansion, bluntly rounding and short- 

 ening of the cutting points, which are quite changed into wide 

 cutting edges. 



I have given on pi. VI., fig. E, a group of central and marginal 

 teeth in a, an outer marginal in c, a marginal in profile in d. 



The allied species L. virgineus differs from fasciatus in having 

 a long blunt cutting point to its central tooth, and by the presence 

 of several true lateral teeth with \ong cutting points, also in the 

 presence of several teeth showing a gradual change from the 

 laterals to the marginals. A full description and detailed figures 

 of its dentition are given by me in Ann. Lye. Nat. Hist. N. Y., 

 XL, 41, pi. III. 



Liguus is nearly allied in its lingual dentition to Oi^thalicus, but 

 in that genus also I have found one species with true lateral teeth, 

 as will be shown below. 



