284 PROCEEDINGS OF THE NATIONAL MUSEUM. voi,.xxiv. 
The character of the lymph may have .somethinj^ to do vv ith sculpture. 
It quite likely varies in density one time compared with another 
according to the proportions of conchiolin or lime; with the first 
maximum, would be greater fluidity and less tendency to set, at the 
expense of diffusion or quick spreading, than when the lime factor 
dominated and made it more viscous. 
As to the proportions of the mantle and its size as related to the 
size of the mouth and other characters we have no certain knowledge. 
It ma}" be that the edge of the mantle is thin and simple; that a series 
of pores occur that are parallel to but not quite at the edge of the 
mantle; that these pores are nearl}" equally spaced as to distance apart. 
In that case we may suppose the two forms of sculpture could be made 
at the same time — that is, the spiral or transverse and the longitudinal — 
the deposition from the mantle's edge forming on the edge of the 
outer lip the longitudinal plicae and the secretions from the pores 
simultaneously deposited forming the lirse. In this case any differen- 
tiation in the size of the lira^ — that is to sa}^ coarseness or fineness- 
would be due to lack of uniformity in the size of the pores, and irreg- 
ularity or differences in distance apart might be attributed to the par- 
tial or absolute congestion of some of them. 
Angulation or shouldering, in some instances amounting to absolute 
distortion, as seen in asymmetry and bulging, are presumably due to 
hypertrophy of the visceral mass, especially the liver, and to entan- 
glement at some time during the period of growth in the vegetable 
matter in the midst of which they live, and the flaring of the mouth, 
as seen in fig. 12, to the close pressing to some flat surface, like a tule 
stalk or the stem of some other aquatic plant. 
The register number of the shells, figured 1 to 18 inclusive, is 
U.S.N.M. 1:7851. 
OTHER SPECIES OF THE AMNICOLID^. 
There will generally be found in an}' collection of the Desert shells 
a small proportion of the following: 
PALUDESTRINA LONGINQUA Gould (Pilsbry). 
Amnicola longinqna Gould, Proe. Boston Society Nat. History, Mar., 1855; Binney 
Land and Fresh Water Shells of North America, Part III, Smithsonian Miscellaneous 
Collections, No. 144, Sept., 1865, p. 87, fig. 173. 
In Dr. Pilsl))y\s revised Catalogue of the Ainnlcolkla^ etc., he 
includes this form in the genus above named. Its presence in the 
desert is inconspicuous when compared with the vast numbers of 
P. protea. Those factors in the environment to which is apparently 
attributable the variability of P. protea^ do not seem to affect 
P. longinqua^ for all of the shells in the latter have a smooth unsculp- 
