206 EOCENE MOLLUSCA. 



genus, the distinctions between some of the groups will, T 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 Pleurotomae, pro- 

 posed by M. Deshayes, into fmiformes and conoidales ; but the fustformes 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 Pleuro- 

 toma through the conoidal forms of that genus into the species of Cone 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 Conidae. So, again, the passage through Lachesis into 

 Murex — that through the shells before referred to with the so-called rudimentary 

 sinus in the outer lip into the true Fiisus ; 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 Drittia, present strong 

 resemblances to the Strombida. 



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, S. Wood, 

 Homes, and the many other authors who have described the mollusca of the newer 



