PROSOBRANCHIATA. 211 
all much roughened, almost decussated, by the lines of growth; the spiral lines on the 
posterior margins are numerous and nearly equal, the two or three nearest the suture 
being rather more prominent than the others. The aperture is of a lengthened, ovate 
form; the outer lip much arched; and the sinus, which is placed in the front part of 
the marginal depression, is deep and moderately wide. Specimens frequently occur 
(var. nanodis, fig. 8 6) in which the shell is shorter and wider than in the typical form, 
and the margins of the whorls not being so much depressed and hollowed out, give a 
nearly conical form to the spire; but the character of the concentric lineation, the 
shape and position of the sinus, and the form of the outer lip, correspond with those of 
the type. 
Other forms also occur which, although presenting differences in the proportions or 
ornamentation of the shells, or in the form of the outer lip, agree in other respects 
so closely with the present species, that they appear to me to be merely varieties 
of it. 
In the first of these forms (var. credrilinea, fig. 8f), the shell is smaller and nar- 
rower, the spire more pointed and obscurely tuberculated ; the spiral lines are slender, 
numerous, and deeply cut by the sharp, prominent lines of growth, and the margins of 
the whorls are finely plicated. In the next form (var. ¢uéerculata, fig. 8 c—e) the spire is 
more slender; the concentric ornamentation resembles that of the variety crebrilinea, 
but the whorls are angulated, and present a single row of slightly oblique, oblong 
tubercles, which are continued, in some instances, even on the last whorl of the fully 
formed shell. The third variety (var. /atimarginata, fig. 8 g,h) resembles the preceding 
variety in the proportions of the shell, and the angular and tuberculated whorls, but 
the posterior margins are widely spread out, and pressed against the preceding 
whorls, which are covered by them almost up to the tubercles. In all the last three 
varieties, the outer lip is not so much arched as that of the typical form. 
This species so closely resembles one from Tortona (P. granulosa, Bon.; P. Sis- 
monde, Bell.), that it is difficult to separate the two. In the Italian shell, the whorls 
appear to be more numerous, and the posterior margins present from three to five 
rows of bead-like granulations near the suture; but, judging from the figure and 
description given by Bellardi, the two species appear to agree in the form of the 
whorls, the character of the spiral ornamentation, the condition of the outer lip, and 
the position and shape of the sinus. The differences pointed out are scarcely sufficient 
to justify the separation of the present species; but without an actual comparison of 
the shells themselves, I do not venture to identify a form from the older eocene 
deposits in this country with one from the pliocene formations of Italy. 
Size.—Of the type; axis, 2 inches and a half; diameter, 10-12ths of an inch. Of 
the variety fwberculata ; axis, 1 inch and 7-12ths; diameter, not quite half an inch. 
Localities—The species is found, in all its varieties, at Highgate; the variety 
crebrilinea also occurs at Potter's Bar and Shenfield; and the variety fwberculata at 
