326 EOCENE MOLLUSCA. 
several, the columellze of which present an oblique, obscure, fold-hke callus, similar to 
that which characterises the columella of P. cataphracta (Brocc.) and P. Delucii (Nyst.); 
but in these instances the character is due to a callosity occasioned by the contortion 
of the columella. The fold presented by Bellardi’s species (Porsonia prima) is nearly 
transverse, sharp, and well defined, and appears to be a true columellar fold, quite dis- 
tinct in character from the callosity to which Ihave referred. That author, therefore, in 
defining his genus, has carefully excluded those species which present merely a callus- 
like prominence, distinct from and not to be confounded with the elevated and clearly 
defined fold, characteristic of Borsonia. 
Subsequently other Pleurotomoid shells, from the Eocene formations in the environs 
of Pau, were noticed by M. Rouault, on the columella of which two or three folds 
were found; and that author, misapprehending apparently the true character of the 
fold in Borsonia, which he says appears to be only a small ridge on the left margin, and 
not a part of the columella, proposed the genus Cordieria for such Pleurotomoid shells 
as possessed two folds or more on the columella. The careful manner in which 
Bellardi has restricted the character of the fold in Borsonia, and the absolute want of 
any distinct generic value in the presence of one or more additional folds, appear to 
render the further division of the group proposed by Rouault unnecessary. 
The genus Borsonia is not admitted by D’Orbigny ; but, without entering into the 
question whether and under what conditions the presence or absence of clearly 
defined folds on the columella is to be regarded of generic value, it is convenient, at 
all events for the present, to retain Borsonia as a well-marked section of a genus 
already overcrowded with species.* 
Two representatives of the genus are found still living, both inhabitants of tropical 
seas. In addition to the species noticed by Bellardi, three other Borsoniz from Biaritz 
and Bos d’Arros, in the neighbourhood of Pau, have been recorded by Rouault. With 
one of these (B. Biaritzana), a species from the middle Eocene beds in England, de- 
scribed by Sowerby as Pleurofoma curvicosta in Dixon’s ‘Geology’ &c., of Sussex, corre- 
sponds so closely that I have not ventured to retain it as distinct. Certain shells from 
Grignon and Parnes, described by Deshayes as Plewrotoma nodularis, present two folds 
on the columella, and will therefore belong to the present genus ; and the description of 
a sixth species (Borsonia sulcata), from the upper Eocene beds, has also been given in 
Professor E. Forbes’s memoir on the tertiary fluvio-marine of the Isle of Wight. To 
these two more species are now added. 
* The genus in question establishes a passage between Pleurotoma and Fasciolaria, or rather Turbinella, 
the folds being higher up the columella and more transverse than those of Fasciolaria, and approaching 
more nearly in position and character to those of Turbinella. 
