294 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxiv. 
VARIATION IN FORM. 
The Physas of the desert, however affected by the environment, in 
the matter of sculpture are absolutely wanting in the characters so 
conspicuous in the Paludestrmce. Evidence of hybridization is at 
once suggested when a large number of individuals is compared. Dis- 
tortion and pathologic deformit}^ are of frequent occurrence, and are 
exhibited in the strong shouldering of the basal whorl and the bulging 
of the latter, which is not uncommon, as in certain California species 
of L'viinuca from sweet waters. This aspect of deformity may be 
attributed to abundance of food supply, resulting in hypertrophy of 
the viscera. The surface of the shells may sometimes be uneven and 
the incremental stria? be coarse, as the whole is heavier than is usual 
in the shells of this genus from ordinary localities or stations. These 
characteristics are apparently due to the environment, but sculpture 
in the proper sense is not manifest. The excess of mineral matter in 
the waters is exhibited in greater solidity and a sturdier growth. 
The Physas of the Great Basin from the extreme north to the Col- 
orado River and l^eyond show a notable tendency to shortness of spire 
and shouldering or symmetrical bulging and flattening of the upper 
part of the basal whorl. These characters, when pronounced, approach 
distortion. Normal or typical individuals of /"*. hninerom. exhibit 
these features moderately. P. virgata Gould, from the River Gila 
and near San Diego and at Los Angeles, a not remote neighbor of P. 
/lumerosa, which may be regarded as a less chunky aspect of the latter, 
show these peculiarities still less. Both I\ /lefe/'ostrojiha and P. gyrina^ 
if these may be called different species, in some localities within the 
area above mentioned show moderate tabulation of the basal volution, 
with spires of variable height. P. Jordi., sparsely represented in the 
Great Basin, is a conspicuous illustration of the low-spired ventricose 
body whorl type. 
In this connection attention is called to figs. 1 to 10 on Plate XXIV, 
and the modification of the shell through hypertroph}^ is fondbly sug 
gested b\" some of the figures on the same plate. 
A glance at Plate XXIII and a comparison of the figures therein 
exhibits the range of variation from the low-spired, rather chunky 
shells of P. huinerosa 1 and 10 to the elongated forms {approaching 
P. gyrina) P. virglnea 53, 55, and 56, while 18, 23, and 26 may be 
assigned to P. me.ricana. In fig. 29 we have a typical humerom. Figs. 
51 and 5-1 may be regarded as robust examples of P. heterostropha^ 
while hybridization is suggested by many of the intermediate forms 
not specified above. 
Cooke gives several figures ^ illustrating the effect of salinity upon 
LimncBa jjeregra and Z. stagnalis from the salt marshes near the sea 
' Cambridge Natural History, III, 1895, p. 85. 
