186 CLASS III. GASTEROPODA. ORDER VII. PECTINIBRANCHIATA. 
PLEUROTOMA*, Lamarck. 
Testa turrita, fusiformis, spira acuminata; columella levi, in canalem 
rectum, nunc elongatum, nunc breviusculum, desinente ; apertura 
ovali, labro simplici, acuto, posticé vel sinu, vel fissura emarginato. 
Operculum corneum, infra acuminatum. 
The Pleurotome, as stated in our observations on the preceding genus, 
were separated by Lamarck from that portion of the Linnean Murices 
which were set apart by Bruguiére for the formation of his genus Fusus. 
He was tempted to remove these mollusks in consequence of their shell 
having the outer lip always more or less deeply fissured or emarginated 
towards the upper part, just at its junction with the last whorl ; and 
upon observing the wide difference that exists in the length of the canal 
in different species, he was induced to make a further subdivision, by 
referring those in which the canal is comparatively short to another 
genus, under the title of Clavatula; this, however, at the suggestion of 
Cuvier, he very judiciously abandoned. The Pleurotomz are numerous 
in species, and constitute a very beautiful and interesting genus; their 
shells are for the most part of an elegant fusiform shape, and may be readily 
distinguished by the notch or fissure in the upper part of the lip, without 
reference to the length of the canal. This slit is undoubtedly left by a 
corresponding fissure in the edge of the mantle ; it may be noticed in all 
stages of growth, and the manner in which it is filled up as the shell 
advances may be generally traced throughout. 
The shell of Pleurotoma may be further described as having a regularly 
acuminated spire, and a smooth columella, ending in a straight canal, 
which varies considerably in length; the aperture is oval, simple and 
acute, and, as we have already remarked, is always emarginated or slit at 
the upper part. The operculum is horny and acuminated. 
* The word Pleurotoma, from zhevpoy, side, and rou) (Dor. ropa), slit, is unquestionably 
feminine ; Philippi uses it in the neuter. 
