Bihliographical Notices. 365 



district has been considerably modified since certain portions of the 

 gravel were spread over it. 



In speaking of tlie geological position of the gravel, Mr. Prcstwich 

 enters into some interesting details respecting the characters and 

 hahits of the Mammalia and Testacea belonging to this period, and 

 points out the known, but singtdar and snggcstive fact, that whereas 

 the larger iuunl)er of the species of the great Mammalia have become 

 extinct, a large proportion of the friable and delicate shells which 

 then frecpiented the land and rivers of the same countries which the 

 mammals inhabited, have lived through the various geological changes 

 which have since succeeded. 



The second lecture describes the structure, age, and organic remains 

 of the London Clay. Although immediately underlying the gravel, 

 yet the two deposits are separated by a wide interval (geologically 

 speaking) — an interval during which numerous marine and estuarine 

 strata were deposited, both in this country and on the continent, and 

 even the mighty Alps attained some, if not the entire portion of their 

 elevation. The London Clay is treated of in regard to its height 

 and extent, — its superposition and relative age, — the description of 

 the character and affinities of the animals and plants which flourished 

 during the period at which the formation was accumulated, — the 

 thickness of the strata, and the number and extent of the succes- 

 sive zones of animal life, — the mineral character and structure of 

 the mass, — and the arrangement of the sediment; "so as to form 

 some opinion as to the probable nature of the climate, the depth of 

 the seas, and the position of land and water at that period of time." 

 This lecture evinces much labour and research ; first as regards the 

 principles by which the geological history of past times is deciphered, 

 and secondly, as to the variety and interest of the organic remains of 

 the London Clay. We recommend an attentive perusal of this por- 

 tion of the work, as treated in a novel, instructive, and pleasing 

 manner, and in which the author acknowledges the sources of his 

 information respecting the fossil remains of the formation, and com- 

 j)ares their numerical proportion with those occurring either in 

 Britain or elsewhere. These are treated of in an ascending order. 



Commencing with the lowest or most simply organized creatures, 

 the Fornminifera, a tribe which swarm in our present seas, and form 

 the food of moUusca and iishes, the author states that ten genera 

 have been noticed, including many species. There are ten species of 

 Corals, — none, however, belonging to the reef-building forms. Of 

 Echinoderms, or the Sea-urchin tribe, there are seventeen species, 

 some of which, as the Ophiura Wetherellii, must have been abundant 

 and quietly entombed in the muddy sediment, " since INIr. Wethcrell 

 found at Highgate a septaria about 3 feet in diameter, of which the 

 surface was literally covered with hundreds of these delicate radiated 

 creatures in a fossil state." 



Although numerous, only eight species of Crustacea had been de- 

 scribed at the time of Mr. Prestwich's lectures ; but the recent publi- 

 cation by Prof. Bell* indicates 17 species; 4 of Macrura, 10 of 



* Palscontographical Society, 18.56. We cannot but regret that the 



