136 Zoology of Colorado 



gill-breathing condition of the fishes; consequently the whale 

 still breathes air, and can be drowned by being deprived of it. 

 Among the arthropods, we have such forms as water beetles and 

 water spiders, which breathe air, but carry it with them in the 

 water, in the form of a bubble attached to the body, or beneath 

 the wing covers. So also with the snails, for the typically terres- 

 trial air-breathing (pulmonate) type has given rise to a large group 

 of aquatic forms which still breathe air, though living in the water. 

 The so-called lung of these animals opens by a single aperture 

 beneath the edge on the mantle, which of course is a very different 

 thing from the internal lungs of terrestrial vertebrates, the latter 

 originating as outgrowths from the alimentary canal. In the 

 fresh water lung-breathing snails, individuals may be seen coming 

 to the surface to get a supply of air, just as a whale has to do. 

 Nevertheless it is sometimes possible for these snails to live 

 without access to the atmosphere, for it has been observed that 

 some of them live at great depths in the Lake of Geneva, and 

 absorb the necessary oxygen for their sedentary lives from the 

 water. It appears that these species (belonging to the genus 

 Lymnaea) are able to absorb oxygen not only through the surface 

 of the lung but also through the surface of the body generally, 

 the skin being sufficiently thin. Possibly similar snails will 

 eventually be found in the depths of some of our cold lakes, which 

 have not as yet been sufficiently explored. Another conspicuous 

 difference between these two groups of our freshwater snails is 

 found in the fact that the gill-breathers possess an operculum, 

 which the lung-breathers lack. This operculum is a sort of a lid 

 which, when the snail is within the shell, closes the aperture or 

 so-called mouth. In some of the sea-snails, the operculum is 

 very thick and heavy, but in our kinds it is very much thinner, 

 and of a horny consistency. 



The operculate snails of Colorado are rarely seen; in fact it 

 appears probable that there is only one living species, the small 

 coiled subglobose Valvata sincera of Say, with a diameter of five 

 or six millimeters. This has been found in the San Luis Valley 

 and in Rio Blanco County. Several other freshwater oper- 

 culates occur in Utah. It is evident from the fossil operculates 

 found in our Tertiary and Cretaceous rocks that Colorado was 

 once inhabited by a great variety of these snails, some of them 



