194 The Subgeneric Groiqys of Naticopsis. — Keyes. 
form at least two and possibly three more or less well defined 
groups. These sections differ so greatly in several important 
particulars that actually they should be regarded as generically 
distinct. Meek and Worthen ^, recognizing the fact, proposed 
to establish three subgenera ; but their subdivisions were based 
upon surface ornamentation. The three sections were : 
Naticopsis proper ; a group typified by Nerita subcostata 
Goldfuss, but not named ; and Trachydoniia, including N. 
nodosa^ M. & W., JS. hollidayi M. & W. and Littorina iwAeeZ- 
eW Swallow; besides two European species. In Naticop>sis 
proper, as represented by the typical forms, and by the major- 
ity of American species referred to the genus, the shells are 
relatively thin ; the spire very short ; the outer lip extremely 
thin and sharp ; the inner lip also thin and slightly depressed ; 
the last volutions generally more or less distinctly flattened 
or concave on the upper half and marked towards the suture 
by numerous small, short, equidistant costce parallel to the 
lines of growth ; the surface otherwise glabrate. In contra- 
distinction the shells of Trachydomia are massive, with the 
spire larger and more elevated ; the outer lip very thick, but 
abruptly becoming sharp ; the columella very heavy, the 
calosity thick and greatly extended ; the volutions shallowly 
channeled along the suture ; and the surface covered with 
numerous small equidistant nodes. A single North American 
species — Littorina wheeleri Swallow — seems to be referable to 
Trachydomia; the other three forms described under the 
genus T nodosa M. & W., T hollidayi M. & W. and T. 
nodulosa Worthen being at present regarded merely as more 
mature individuals of Swallow's species. A comparison of an 
extensive series shows that within certain limits the shells of 
this group are quite variable. The calosity in some speci- 
mens is much more extended than in others ; while numeric- 
ally the surface nodes vary greatly in different individuals, 
and become much larger and more widely separated as, with 
age, the shell increases in size. 
Another group closely allied to Naticopsis is Isonema; the 
type of which now appears to belong to the former genus. As 
originally established by Meek and Worthen " it was regarded 
as a subdivision of Holopea; and had for its characteristic spe- 
^ Geol. Surv. 111., vol. ii, p. 364. 1866. 
*Proc. Acad. Nat. Sci., Phila., 1866, p. 251. 
