THE NAUTILCS. 27 



and hardens. In some species the exudation is continuous, or nearly 

 so, while in others it seems to he deposited at intervals. When the 

 deposit is frequent and small, or continuous, the lines of growth will 

 be fine and the surface smooth and shining, as in P. heterostropha 

 Say. When made at longer intervals, and large, the lines will be 

 coarse, as in P. gyrina Say. 



Structure refers to the general plan of the shell, but more particu- 

 larly to the manner in which the whorls are joined together. In 

 some forms, like P. ancillaria Say, the whorls lap over each other 

 like shingles on a roof; in others, like P. forshfyii Lea, they rest 

 against each other. This distinction is constant in all species, and 

 to a great extent is carried through the varieties. 



Sculpture is the markings on the surface of the shell, and consists 

 of the lines of growth, strire crossing the lines of growth, as in P. 

 oleacea Tiyon, ribs, as in P. costata Kewcomb, and such other mark- 

 ings as may be found constant in a species. 



It must be remembered that while these characters are always 

 found in typical members of species having one or more of them, 

 they become modified in the varieties and forms, and the greater the 

 departure from the type the greater will be the modification. 



Species are supposed to be founded upon permanent characteristics 

 which are not influenced by change of location or environment. 

 Varietal changes are many and frequent, and it is doubtful if a single 

 variety described forty years ago can now be found living and identi- 

 fied, while species should be found in the habitat of the type with 

 but slight variations in its characterization. 



No member of the animal kingdom is so susceptible to local influ- 

 ences as fresh water snails. Every change of environment changes 

 the Physa, and so long as environments continue to change new forms 

 will continue to occur; but they will not necessarily be new species 

 nor varieties, but new forms of the same species, having the same 

 characteristics upon which it w r as founded, vsith some of them more 

 or less modified. 



Another cause for variations is the probably frequent occurrence 

 of cross breeding, producing hybrids. It is often that forms are pre- 

 sented to conchologists for identification having some of the charac- 

 teristics of two different species, neither of which is sufficiently strong 

 to control, and the life history not being known, they are described 

 as new species. The forms being hybvids, when the crop disappears 



