192 Proceedings of the British Association for [Sept. 



He observed that such was the progress which the study of 

 organic remains had made, that no less than one hundred 

 species were now known to naturalists ; and of these, about 

 thirty- four species had been found in England. 



Shells of this genus are confined to the chalk, oolite, and 

 lias, and the results which their study affords, contrast re- 

 markably with the negative indications deduced from an 

 examination of the fossil Astaci. One division characterized 

 by a little swelling at the apex, and possessing a lateral 

 fissure, was confined to the chalk. The species which were 

 obtusely mucronate are found in the green sand. The 

 species with a groove in the back are found in the middle 

 oolite : Those with a lateral groove, in the lias, and lower 

 oolite ; and those species which are destitute of a groove 

 are confined to the lias. From these remarks, it appears 

 that not only are the species of Belemnite confined to cer- 

 tain strata, but that even certain natural divisions of the 

 genus are found together in the same beds, and in no others. 

 Another curious remark is, that species which are common 

 in the chalk of the Continent, are rare in the chalk of 

 England, and vice versa. 



These remarks were followed up in an admirable manner 

 by M. Agassiz, who, from a study of the remains of this 

 difiicult genus, clearly demonstrated that the shell was an 

 interior one, analogous to the bone of the cuttle fish, and ' 

 not an exterior shell, as is generally imagined. 



6. Lieutenant Denham, R.N. , one of the oflftcers attached 

 to the Ordnance Survey of Great Britain, exhibited ^ map 

 illustrative of the estuaries of the Dee and Mersey. He 

 was one of the first to discover, along with his brother 

 ofiicers, an important channel in the Mersey, after various 

 ineffectual and expensive efforts had been made to improve 

 the navigation of that river, an object of the greatest im- 

 portance to Liverpool. The channel runs North and South, 

 at right angles to the tide which washes over it. Lieutenant 

 Denham called the attention of the Section to the half tide 

 level, as a means of determining the level of the sea and the 

 land, the main level, or half tide being always constant, the 

 high tide being variable from various causes. In illustration 

 of his views upon this subject, he exhibited a chart of the 

 estuary of the Dee and Mersey, and a tide table, calculated 



