WARBURTON ON PLASTIC CLAY SEPTARIA. 173 



The following is the description of the shells, by Mr. E. Forbes : 



1. Paludina. 



A species with 5 ventricose whorls which appear to have been 

 slightly striated by lines of growth; the largest perfect specimen 

 measuring 1 inch in height by half an inch in greatest breadth of 

 the body whorl. 



I can find no character by which to separate this fossil from the 

 existing Paludina vivipara ; some of the southern varieties of which 

 it closely resembles in external form. That species is very widely 

 distributed, at present being common to a great part of Europe and 

 Asia. I am not aware of its ever having been previously noticed 

 as an older tertiary shell, but its allies, P. achatina and P. unicolor, 

 are both recorded tertiary species, having a wide range both geolo- 

 gically and geographically; the identity of the living species with the 

 fossils discovered by Mr. Warburton and Mr. Simms is thus rendered 

 the more probable. I regard this species as quite distinct from the 

 Paludina fiuviorum of the Wealden, which was confounded with 

 P. vivipara, a confusion which has led to several erroneous state- 

 ments and reasonings. 



2. Unio. 



An ovate, very inequilateral, depressed species, wrinkled trans- 

 versely, growing to a length of between 2 and 3 inches. 



The extreme difficulty of determining living species of Unio 

 must render us very cautious in giving an opinion on a fossil. The 

 remains, though not imperfect of their kind, and certainly those of 

 Unios, are not sufficient to warrant the bestowal of a specific ap- 

 pellation on them. They are, however, probably distinct from any 

 of the recorded British tertiary species. E. F. 



I have only to add in conclusion, that in the Paris basin, which 

 lies to the north of Paris, in the Departement de l'Aisne, fresh-water 

 shells belonging to the same genera occur in the plastic clay series, 

 accompanied by lignite ; and among the fossils of one of these 

 beds, M. D'Archiac enumerates four species of Paludina, and an 

 undetermined species of Unio. Mons. Charles d'Orbigny also, in a 

 section of the beds in the Paris basin, intermediate between the 

 calcaire grossier and the chalk, near Meudon, enumerates plastic 

 clay, lignite with Paludinae and Anodontas, and a conglomerate 

 containing fresh-water shells. 



This point of correspondence between the plastic clays of Eng- 

 land and France, viz. their containing several of the same genera, 

 at least of fresh-water shells, besides Cyclades, which have been be- 

 fore noticed as common to the two, and Cerithia, and other shells, 

 inhabitants of brackish water, I have thought of sufficient interest 

 to make the subject of a notice to the Society. 



