PROSOBRANCHIATA. 207 
tertiary deposits. In the Western hemisphere, which presents a large proportion of 
the living species, the genus does not appear to have had so large a development ; 
twenty-three species only from the tertiary deposits in the United States have been 
described by Conrad and Lea, and three species from the newer tertiaries of Chili have 
been described by Sowerby. The genus is largely represented in the English eocene 
fauna, but as yet comparatively few species have been described or identified. 
The shells of the Plewrotome appear to have been peculiarly subject to modification 
by external conditions, and, as De Blainville has remarked, it is “apparently with 
them as with the Cerithia, the Ammonites, and other genera which contain many 
species ; each locality presents different forms.” 
Section 1. Shells fusiform. 
A. Sinus in the posterior margin of the whorl. 
a. Canal elongated. 
No. 133. PLEuRoToMA sTENA, /. H. Edwards. Tab. XXV, fig. 4a, 6. 
P. testi elongato-fusiformi, angustd, sub-turritd, fasciis et filis spiralibus, lineis incre- 
menti decussatis, omnino tectd; spird productd, apice acuminato : anfractibus convexius- 
culis, angulatis, obtuse carinatis, postice sub-concavis, ad suturam crenulatis ; filis concen- 
tricis numerosis, inequalibus ; anfractu ultimo antice gradatim attenuato et in canali longo, 
recto, eveunti: apertura elongato-ovali ; labro vie arcuato ; sinu labrali angusto, profundo 
in margine collocato. 
Shell elongate, fusiform, narrow, ornamented with numerous spiral bands, the 
spaces between which, as well as the surface between the shoulder and the suture, 
and sometimes even the spiral bands, are covered with numerous fine, thread- 
like, unequal, raised lines, decussated, or rather roughened, by the lines of growth; 
the spire, which is formed of seven or eight volutions, is much produced ; the whorls 
are slightly convex, sharply angulated at the shoulder; the posterior margins de- 
pressed and slightly thickened at the edge, where one or two raised lines, stronger than 
the others, and crenulated by the lines of growth, run round the suture; the space 
between the suture and the shoulder is concave, giving somewhat of a turreted aspect 
to the spire. The spiral bands are irregular, narrow, flat on the surface, sharp edged ; 
the posterior band runs round the shoulder, forming a blunt keel, and the space 
between it and the band immediately in front of it is wider than those between the 
other bands, and is concave; the bands, as they approach the anterior part of the 
shell, become closer, narrower, and less prominent, while, on the other hand, the con- 
centric lines become stronger, more elevated, and more distant, until the two blend 
together, and form the round, coarse, raised lines which cover the base of the shell 
