48 



R. pygmoea is the Brachiopod most conspicuously present, foUo-wed by 

 T. ghhulina, and Leptence. 'That characteristic shell, Posidonomya Bronni, 

 occurs in the blue clay associated with Nuculoe, under the Leptoena bed, 

 but is comparatively rare in the shaly portions. It is a question whether 

 pressure and percolation may not have obliterated so thin and fraH a shell. 

 Still, on the other hand, we find in the Oxford clay impressions and traces 

 of the same bivalve beautifully distinct. Nucula inflexa, of Quenstedt, is 

 rare ; it is a charming sheU and has more the external appeamce of Leda, 

 posteriorly at least. Here it is the only representative of the Arcades ; and 

 suspicion is not wanting that this Nucula is identical with the Nucula 

 Sausmanni of Eoemee, cited by Mr. Ltcett, as procured by him from an 

 exposure of the Upper Lias shales at Ifailsworth, since covered up. Mytilus 

 gryphoides is a well-marked fossil of the Limestones, though ranging above 

 and below them. It is not unapt to be confounded with the Inoceramus 

 dubius of SowERBT, but no two fossils can be more distinct either in outline 

 or growth. Eriefly, the smooth tumid or rather pouched form clearly 

 belongs to the Mytilidoe, and is no Irioceramus. It is fair to state that 

 Oppel entertains a faint doubt (v. Juraformation, § 32, p. 261). With 

 more time and favourable opportunity, I feel convinced that I shall be 

 able considerably to increase my list of fossUs ot the communis zone at 

 Churchdown. This addition will be chiefly in the minuter forms of the 

 Brachiopoda of the Leptoena bed ; Thecidia for example, which on account 

 of their diminutive size, cannot be safely and readily defined without 

 ample time and patience. 



Keference may here be made to the dwarfed size of the organisms of the 

 zone, as though starvelings of a larger race, that came from a distance, and 

 struggled for existence. Whether the waters of our liassic estuary were 

 deficient in a normal per centage of salt is a view not unworthy our 

 consideration. Such a cause would adequately suffice to degenerate the 

 moUusca inhabiting them. Eussian naturalists tell us that while oysters, for 

 instance, are found in the Mediterranean, the Atlantic, the North Sea, and 

 the northern part of the Cattegat, they do not occur in the Baltic Sea, and 

 refuse to be naturalized there. To accord with this, the oyster cited in my 

 list, is almost a solitary example, not an inch in diameter. The waters of 

 the Mediterranean contain 3-7 per cent of salt; those of the Atlantic 3 to 

 3-6 per cent; the north of the Cattegat 1-8 to 2 per cent; whUe the 

 saltest part of the Baltic yields only 1.7 per cent, of salt. In Professor 

 E. Poebe's "Travels in Lycia," 1847, it is mentioned that a gradual 

 change in fossil marine species of Mollusca was observed by the writer in 

 some coast sections of the Island of Cos. The deviations in form 

 evidently arose from the influx of fresh water. Every Algologist 

 must have noticed the stunted dimensions of such marine Algoe as 



