Mr. S. V. Wood's Caialoyue of Shells from tht Crag. 527 



Vicinity to large towns has a visible influence over vegeta- 

 tion. Around London it possesses a good deal of luxuriance. 

 A cause for this maybe sought in the state of the atmosphere 

 liable to exist among such a crowd of habitations and human 

 beings. Pure air, after being once respired by man, contains 

 about 3'6 per cent, carbonic acid ; but the extreme dilution this 

 must undergo in mixing with the bulk of the atmosphere, ren- 

 ders it unlikely that it will have any visible effect. It is more 

 probable that the immense quantity of carbon, in an extremely 

 fine, light, and divided state, which escapes in smoke after 

 combustion, is a more influential cause. It is now in a con- 

 dition to be suspended, if not dissolved, in water, and can pass 

 readily through the structures of plants ; and the good effects 

 of certain proportions of carbon in a convertible state has 

 been proved by experiment. In estimating the influence of 

 large towns on vegetation, it must not be lost sight of, that 

 combustion also gives rise to some of a deleterious tendency. 

 Sulphurous acid is produced in a sufficient quantity to impair 

 the functions of plants in a sensible manner, and even the bad 

 effects of an extremely minute proportion have been noticed. 

 Those plants Avhich are observed to prefer the vicinity of 

 clustered habitations have then, most probably, some con- 

 nexion with the resulting state of the atmosphere whence they 

 derive benefit ; some may receive positive benefit or stimulus 

 from it, and others be equally injured. 



LVI. — A Catalogue of Shells from the Crag. 

 By S. V. Wood, Esq., F.G.S. 



[Concluded from p. 4G2.] 



Class GASTEROPODA. 



Ord. Phytophaga. 



Cor. Cra[i. Red Crag. Mam. Crag. Recent. 



1 . Ca])ulus ungaricus, de Montf. (Patella ungarica, Mont. Test. Brit. 



p. 4SG. Patella unguis, var. ft. Min. Con. t. 139. f. 7). 



Ramsholt. | Sutton. [ | Britain. 



This exceeds in magnitude the recent British specimens. My 

 largest fossil iias attained the (transverse) diameter of two inches 

 and a quarter. A very variable si)ecies : some of my specimens arc 

 conical, with the apex nearly central, while others are so much de- 

 pressed, that the apex is on a level with the base jjrojecting beyond it. 



2. — obliquus, n. s. 



I WaltonNaze. | | 



3. — rccurvatus, n. s. 



I WaltonNaze. I I 



