Insect — Limestone. 13 



down into the sea by a river, which also brought down the 

 leaves of ferns and monocotyledons, together with fragments 

 of other plants, possibly dicotyledonous.* That no shells of 

 snails or any air-breathing testacea have been yet obtained 

 must be a mere accident, and we may consider ourselves in 

 this and many other districts as treading on the threshold of 

 discoveries, which may soon make us better acquainted with 

 the contemporaneous flora and fauna of the dry land. 



Until the year 1846, when Dr Dunker described the Weal- 

 den beds of the north of Germany, no well-determined speci- 

 men of the genus Planorbis had been detected in beds older 

 than the eocene ; and the first species of Lymneus, more 

 ancient than those of tertiary date, was found by the same 

 geologist. In the four years which have since elapsed. Prof. 

 E. Forbes has not only met with various species of these 

 genera in each of the three divisions of the Purbeck in Dor- 

 setshire, but has also obtained from the same rocks Valvata, 

 Physa, and Melania in abundance, together with the fruit of 

 Chara, previously supposed to be characteristic of strata 

 newer than the uppermost chalk. You will find that some 

 able palseontologists have been disposed to lean on the absence 

 of pulmoniferous mollusca in all strata antecedent to the 

 eocene, in support of the theory of successive development, 

 arguing that these tribes, possessing as they do a high grade 

 of organization (although by no means the highest among 

 the mollusca), came latest in the order of creation, or were 

 not formed until the tertiary epoch. It is therefore import- 

 ant that we should have made so large an accession of fossils 

 referable to this division, some of the species approaching 

 very closely in their forms to living English shells, in strata 

 of such antiquity ; for the recent researches of Prof. Forbes 

 leave little doubt that the Purbeck belongs to the oolitic 

 type, judging from its intercalated marine shells and echino- 

 derms, whereas the marine mollusca of the Hastings sands, 

 collected by Mr Austen, imply that that division of the Weal- 

 den has a closer afl&nity to a lower cretaceous fauna. Ac- 

 cording to this view, the genera Planorbis, Lymneus, Val- 



* Prof. Buckman, Quart. Journ. Geol. Soc, vol. vi., p. 417. 



