3^50 Prof, Sedgwick on the [May^ 



spot where they are found. Again, in the great argillaceous 

 deposit between Headen Hill and Colwell Bay, and also in the 

 chtf near Hampstead, other marine shells are found in a beautiful 

 state of preservation. The ruinous condition of the chff precludes 

 the possibility of determining their exact arrangement. They 

 appear, however, chiefly to abound in the lower part of the 

 deposit. Without pretending to give a complete list even of the 

 specimens which we collected, it may be proper to enumerate 

 the following genera : 



(1.) MureXj at least two species abound in Colwell Bay. 



(2.) Buccinum. 



(3.) AnciUa suhilataj by no means a rare shell in Colwell Bay. 



(4.) Voluta, resembling valuta spinosa, 



(5.) Rostellaria rimosa. 



The last two mentioned species are certainly rare. There are 

 two or three other houday clay fossils in the marine beds of Col- 

 well Bay : e. g. the murex effossus and murex iiinexus of Brander 

 (Foss. Hant. Nos. 28, 30), and fragments of a species offusus. 



(6.) Natica. 



(7.) Venus. 



(8.) Nncula. 



(9.) Corbulaj a small species is very common in Hampstead 

 Cliff. 



(10.) Two small bivalves (of the genus corbis?) abound in the? 

 upper part of Hampstead Cliff. One of them is also common in 

 Colwell Bay. 



(11.) Mytilus. 



This list, however imperfect, is quite enough for our present 

 purpose. Along with the preceding species are some which are, 

 perhaps, of an ambiguous origin ; and others which are undoubted 

 freshwater shells. A few of the latter specimens were probably 

 drifted into the beds at the time of their formation ; but the 

 greater number may have descended into their present situation 

 by the degradation of the superior strata. As we ascend to the 

 higher parts of the formation, we find innumerable examples of 

 certain genera which do not exist in this chmate, but have been 

 discovered in various parts of the world among the inhabitants 

 of freshwater. The cyclas and potamides are most abundant, and 

 with them are generally associated the melanopsis and nerita^ &c. 

 There are two species of neritcCy one of which nearly resembles 

 the neritajiuviatilis. As, however, the beds abounding in these 

 genera also contain a few marine shells, may they not have been 

 deposited in the brackish waters of an estuary, or m a basin which 

 was still partially affected by a marine inundation? In some 

 beds at the top of the formation, which are exposed by a land- 

 slip in the middle of Totland Bay, all traces of a marine origin 

 have disappeared. The phenomena exhibited by the section are 

 as follows, beginning from the bottom : 



