PROSOBRANCHIATA. 297 
Lineis concentricis medianis granulato-nodosis, wrregularibus, alternatim filiformibus et 
tenioliformibus ; apertura ob-ovali ; labro aliformi ; sinu angusto, profundo, marginibus 
sub-parallelis. 
Shell elongated, slightly turreted, longitudinally plicated, concentrically lined ; the 
spire obtuse, pointed at the extremity, and much produced, being half as long again 
as the aperture; the whorls, eight or nine, exclusive of a small, smooth pullus of two 
turns, are nearly straight-sided and angular on the shoulders, where they are girt bya 
narrow, slightly elevated band, which is furnished with a single row of small, close-set 
tubercles, in some few specimens rounded or oblong, but most generally narrow, 
curved, and pliciform ; the posterior margins are narrow, concave, and bordered round 
the suture by a band, formed of two or three concentric lines more prominent than the 
rest, and presenting a series of small, oblong, or roundish tubercles ; the last whorl is 
much contracted in front, where it forms a rather wide, short, but distinct, and oblique 
canal, notched at the extremity. The longitudinal plications are numerous, sharp, 
and curved, and they extend to the very base of the shell; the concentric lines 
over the shoulders and posterior margins are undulating, regular, even, rather 
thick, and separated by perspicuous furrows ; over the middle and front parts of the 
whorls they are depressed, irregular, and unequal, narrow ribbon-like lines alternating 
with fine thread-like lines; at the points where they are intersected by the longi- 
tudinal plications, they rise into oblong tubercles, or become coarsely granulated, as 
the concentric line is more or less broad. The aperture is nearly oval; the outer lip 
wing-shaped, projecting most towards the posterior extremity, very thin and sharp on 
the edge, and smooth within ; and the sinus, which is on the shoulder, is narrow and 
very deep, with nearly parallel margins. 
The roughly granulated surface of this Plearotoma has somewhat of the aspect of a 
coarse file, from which resemblance the specific name is taken. The species presents 
a very close analogy with the variety recorded by Deshayes of P. dicatena (Lamk.) 
In the French shells, however, the margins of the whorls are more depressed, and the 
spire, consequently, is slenderer, and tapers more regularly ; the tubercles on the 
shoulders are longer, more prominent, and more distant, and the anterior canal is 
wider. In the shape and position of the sinus, as well as in the condition of the 
sutural tubercles and of the transverse sculpture, the shells agree, except that the 
decussation caused by the more prominent lines of growth have given to the English 
shells the scabrous aspect which characterises them. It may, perhaps, be considered 
as merely a strongly marked variety of Lamarck’s species. 
Size—Axis, 10-12ths of an inch; diameter, 3-12ths of an inch. 
Localities —Barton, Alum Bay (Stratum, No. 29, Prestw.), and Highcliff, at all 
of which places it is somewhat rare. 
38 
