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EOCENE MOLLUSCA. 



several, the columellas of which present an oblique, obscure, fold-like 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 I have 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 Porsonia, 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 w T ant 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 Borsonise from Biaritz 

 and Bos d'Arros, in the neighbourhood of Pau, have been recorded by Rouault. With 

 one of these (P. Piaritsana), a species from the middle Eocene beds in England, de- 

 scribed by Sowerby as Pleurotoma 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 Pleurotoma nodularis, present two folds 

 on the columella, and will therefore belong to the present genus ; and the description of 

 a sixth species {Porsonia 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 Turbiuella, 

 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. 



