154 



THE BRITISH NATURALIST. 



[July 



gypsum of the district. Herr Julius Hazay states that he has 

 verified by actual experiment that L. peregra and L. ovata 

 (L. peregra var. ovata) may be produced at will by simply placing the 

 ova under suitable conditions, typical 2^^^^gi'^^ is produced from the 

 ova of ovata if placed in hard running water, and ovata from the ova of 

 peregra by an opposite treatment. Deficiency of Moisture. - -Dry regions 

 are tenanted mainly by mollusks with thick and uniformly coloured 

 dull-white shells. The Rev. Canon Tristram remarks that the shells 

 found in the Sahara are much thicker than those of the same species 

 in more temperate parts, and he is disposed to regard this modification 

 as a means of preventing evaporation in so dry a climate, the white 

 shell IS considered desirable as absorbing the least possible amount of 

 heat. Mr. Taylor remarks that the uniformly dull white specimens of 

 H. evicitorum, H. virgata, &c. in our own country agree with the 

 desert forms in being thicker than usual, and quotes a remark of 

 Strobel, that in Moravia the thick uniformly white variety of H. 

 virgata is exclusively found on the open cultivated lands, and 

 attributed their peculiarities to dryness and warmth. M. Morch, 

 writing on the land shells of the Nicobars, mentions that in almost all 

 the species, two forms, a larger and a smaller have been met with^ 

 and that the larger was found by the old collectors, whilst the smaller 

 occurs now, he attributes this to a diminution in the humidity of the 

 climate. Abundant Moisture, according to Herr Dietz, prevents the 

 formation of coloured bands in H. hortensis, and as the result of his 

 observations, says that the albino specimens are more common in wet 

 years, and that the specimens with coloured bands have the growth of 

 the last wet year not coloured. Light. — The absence of light appears 

 to favour, in those species which habitually live in shade and retirement, 

 the production of shells having a uniform but obscure colour, largely 

 composed of animal matter, and often provided with hairs and other 

 epidermic appendages, H. aculeata, H. Jiispida, H. ohvoluta, B. 

 montanus, B. obscuvus and the forest-loving American Helices are quoted 

 as examples. " Arboreal species, which are almost necessarily 

 exposed to the full action of air and light, are distinguished by 

 brighter, more vivid, and more varied colouration, than the purely 

 terrestrial species loving shade and concealment. Our variegated 

 Helices — nemovalis and hortensis approach perhaps nearer in habit to 

 arboreal life, living freely exposed on hedges and hedgebanks, and 

 these species are the most gaily coloured of our native shells." Heat 

 — as already mentioned when accompanied with dryness produces a 

 white or whitish shell, but when combined with moisture and rich 

 vegetation it produces some of the most richly coloured and finest 

 shells known. H. pisana in sunny places has the mouth tmged with 

 rose colour which is said to be deficient in less favoured spots. On 



