Geology and Geography. 385 



which have been described as specimens of a fossil Loligo. It 

 results from this fact, that the fibrous cone of Belemnites is ana- 

 logous to the projecting point of the extremity of the bono of 

 the Sepia, that the plate whicli has been described as a Loligo 

 corresponds to the external portion of the plate of the bone of 

 the Sepia, and that the alveolar cone of Belemnites represents the 

 internal portion of the os sepia: ; so that the genus to which we 

 must refer the Belemnites and the fossil Loligo as parts of the 

 same animal, approaches much the genus Sepia, and, indeed, 

 differs only in the relative proportions of the conical and dilated 

 portions of the shell. Professor Buckland is about to publish, 

 in his Bridgewater Treatise, a representation of the specimen 

 which has enabled M. Agassiz thus to unite these fossils, and 

 he will also give the details which prove the connection. But 

 the history of this genus cannot be regarded as complete till we 

 know also the changes which take place in the form of the be- 

 lemnitic cone at different ages, so that we may not be led to 

 establish species from differences depending on growth. 



8. Lieutenant Denham, R. N., one of the officers attached to 

 the Ordnance Survey of Great Britain, exlnblted a map illus- 

 trative of the estuaries of the Dee and Mersey. He was one of 

 the first to discover, along with his brother officers, an import- 

 ant channel in the Mersey, after various ineffectual and expen- 

 sive efforts had been made to improve the navigation of that 

 river, an object of the greatest importance to Liverpool. The 

 channel runs north and south, at right angles to the tide 

 v.hich 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 these views upon this subject, he exhi- 

 bited a chart of the estuary of the Dee and Mersey, and a tide- 

 table, calculated so, that the mariner may know any half hour 

 the banks which might be crossed, thus pointing out what was 

 dangerous, and when it ceased to be so, a matter of great relief 

 to the mariner. Lieutenant Denham complimented the dock- 

 trustees of Liverpool, for their munificent expenditure, in ena- 

 bling him to prosecute his object, and dwelt with particular 

 energy on the great liberality of Sir John Tobin. Lieutenant 



