OF CONCHOLOGY. 237 



sented by tudiculata alone, which does n<5t show such variety, 

 and is near Toivnsendiana in its swollen body-whorl, and few 

 whorls, although otherwise quite different. This alone seems to 

 fill the interval of 140 miles between the range of Mormonum 

 and Traskii. Oarpenteri may be the species living between the 

 latter and ramentosa on the Mt. Diablo range, where a gap of 

 equal length exists, in which no Helicoids lTave been found by 

 recent collectors. It has been mentioned as having characters 

 of both groups. We have no evidence that any of this group 

 extend into the peninsula, Arizona or Mexico. 



Perhaps in connection with their being found only in moist 

 districts and among deciduous trees growing only where there is 

 neither a long duration of the dry season nor a very rocky soil, 

 we may consider the fact that there is no very flat or subangled 

 species of this group. I have, however, observed specimens of 

 arrosa and exarata, slightly subangled, evidently from arrest 

 of development, having formed a lip before completing their 

 growth, the young of both having an angle. 



All of the fidelis group, except those confined to the islands, 

 are found in forests where coniferous trees are either the only 

 or the prevaling kinds, and where the range of the two groups 

 is the same, the trees are of both coniferous and deciduous 

 species. On the treeless islands there are resinous shrubs, 

 which may, perhaps, supply to these animals the place of the 

 coniferous trees. 



Finally, I include among the larger Helicoids Triodopsis 

 loricata, the only Californian representative of a large number 

 of Eastern species, and of T. Mullani. It varies from the char- 

 acters of all the others in having a sort of scale- like reticulated 

 epidermis, as well as in smaller size, than any other species of 

 the United States. As if connected with this small size, we 

 find it rather more widely diffused than the large ones, being 

 the only species found both in the Sierra Nevada and coast 

 ranges. From its smallness it has probably escaped observation 

 in many localities, but I am certain that it does not exist at any 

 of those I have examined south of San Francisco Bay. Like 

 all those of this coast, it is pretty numerous in certain limited 

 stations, often not more than a few rods in extent, while the 

 collector may search in vain for more, over hundreds of square 

 miles surrounding these spots. The presence of lime and, 

 secondarily, of the proper vegetation, especially of trees, are 

 usually the causes of these local colonies. 



The following hypothesis may assist in explaining the present 

 range and intermixture of the larger species, though it can only 

 be sustained by further examination of the physical geography 



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