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 b) 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. crebrilinea, fig. 8/), 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. tuberculata, 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. latimarginata, fig. 8g,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- 

 mondee, 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 tuberculata ; 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 tuberculata at 



