GODWm-AUSTEN : IMPORTANCE OF ANIMAL IN LAND MOLLUSCA. 39 



the peristome, and they leave the greater part of the shell exposed. 

 This shell is spatulate with few whorls (Fig. 5 Da). Next, coming to 

 Girasia (Fig. 3 E and Fig. 5 E) the shell-lobes are still further 

 developed, and are not only in contact but are actually united and 

 grown together, leaving but a small part of the shell exposed to view; 

 the line of junction is marked by a distinct cicatrix, from the place 

 of origin of growth of the lobes near and above the respiratory orifice. 

 It is interesting to note that with this expansion and development of 

 the shell-lobes, the shell of Girasia is a very rudimentary membran- 

 aceous one (Fig. 3 Ea).^ 



Coming in as a link between the genera Austenia and Girasia, we 

 have the large Burmese Girasia respiendens and G. viagnifica, the 

 shells of which are like those of Austenia gigas, the type of the genus, 

 and as another intermediate link, we find a form from the base of the 

 Darjiling Hills not so rudimentary in shell character as presented in 

 Girasia hookeri. 



Lastly, we have the genus Cryptogirasia (F.), in which the shell is 

 reduced to a small calcareous disc (Fa), covered completely by the 

 mantle, and in which no cicatrix can be detected marking the junc- 

 tion of the shell lobes. 



We have here a most beautiful and instructive sequence from 

 species with large substantial coiled shells, to others more simple, to 

 one where the shell is almost lost, while the course of evolution in the 

 direction of the slug-like form is indicated by the growing together of 

 the shell-lobes. Here environment plays a prominent part in pro- 

 ducing the growth of these shell-lobes. Where these species are found 

 in the finest stages of growth, the rain fall is very heavy, and lasts for 

 a long time in the summer months. The heat is very great, and the 

 air is usually in a state of complete saturation, conditions most suitable 

 to the rapid growth of the animal, but less favourable to the formation 

 of shell. Species of Girasia removed from the open air to the drier 

 "Atmosphere of a room, very soon begin to lose vitality, and the 

 extensible shell-lobe soon shrinks and dries up. 



The life history of these eastern molluscs and the variation of form 

 they present foster the views held by several malacologists, that our 

 European genera, Limax, Arion and Geomalacus, are the outcome 

 of a similar phase of development, from shell-bearing genera long 

 passed away. We can imagine (the distribution of land and sea being 

 different) that this part of the world enjoyed a far warmer and moister 

 climate, that with subsequent changes to colder and drier conditions 

 the ancestral genera became extinct, leaving a great gap and scanty 

 evidence of relationship to the genera they are now associated with. 



I next take a case of modification occurring in the internal anatomy 



