206 EOCENE MOLLUSCA. 
genus, the distinctions between some of the groups will, I think, be found to be difficult 
of practical application; nor will the groups proposed embrace all the forms which 
occur in the English eocene fauna. The employment of a few broadly marked 
characters, which the eye can readily seize, will afford, in fact, more effectual aid to 
the student; and with this view I have adopted the division of the Pleurotome, pro- 
posed by M. Deshayes, into fusiformes and conoidales ; but the fusiformes I have divided 
into two sections, distinguished by the position of the sinus, a prominent and unvarying 
character; while the size and, to some extent, the shape of the sinus are subject to 
modification. The first section will comprise the species in which the sinus is placed 
in the margin, that is to say the space between the suture and the shoulder, or widest 
part of the whorl; the second section will embrace those in which the sinus is placed 
on the shoulder of the whorl. Each of these sections will be sub-divided into two 
groups, respectively consisting of the species having the canal produced, and the 
species in which the canal is short or indistinct. 
The genus Pleurotoma is one of peculiar interest ; it seems to form a central group, 
in which either the animals present close affinities with those of the neighbouring 
genera, or the shells, radiating through aberrant forms in which the typical characters 
are prominently retained, present striking analogies with those of apparently distant 
genera; analogies which, if not suggestive of affinities, at least show the repetition of 
similar forms in dissimilar groups. Thus the passage from the true fusiform P/ewro- 
tome through the conoidal forms of that genus into the species of Cove forming the 
section Conorbis, and so into the true Cones, is a transition so gradual and so 
perfect as in itself to afford the strongest evidence of the intimate connection of 
the present genus with the Conidz. So, again, the passage through ZLachesis into 
Murex—that through the shells before referred to with the so-called rudimentary 
sinus in the outer lip into the true Fvsus ; and also that through Borsonia into Turbinella 
or Fasciolaria ; while the short posterior canal in the species forming Swainson’s genus 
Brachytoma, and the anterior notch on the outer lips of the Dri/ie, present strong 
resemblances to the Strombide. 
The living species of Pleurotoma are very numerous, upwards of 459, including 
those forming the different sub-genera, having been described: they are found in all 
parts of the world, but principally in the seas of China and Western America, ranging 
in depth from low-water mark to 100 fathoms. In the fossil state they first appear in 
the upper cretaceous strata, from which four species referred to this genus have been 
described by Goldfuss, Sowerby, and D’Orbigny. During the tertiary epoch the 
genus was largely developed; upwards of 90 species, from the eocene formations of 
Europe, have been described by Lamarck, Sowerby, Deshayes, Melleville, and other 
writers ; while from the more recent formations nearly 200 species have been recorded 
by Brocchi, Grateloup, Basterot, De Koninck, Nyst, Bellardi, Sowerby, 5. Wood, 
Hornes, and the many other authors who have described the mollusca of the newer 
