NO. iL'5r,. FOSSIL SHELLS OF THE COLOR A DO DESER T—STEARXS. 295 
of Aral, and our desert Physas exhibit similar, if not as extreme, dis- 
tortion. With the latter, as heretofore intimated, the abnormalities 
are quite as likely to l)e due to visceral hypertrophy through overfeed- 
ing- as to the salinity of the waters. 
To ilkistrate any species of Physa by a single ligure is, in the present 
state of our knowledge of the susceptibility of the mollusks of this 
family to environmental influences, quite an absurd thing to do. 
While the shells of a colony of any one species inha])iting a spring or 
pool of limited area might be nearly or quite uniform in size, shape, 
texture, and color, aiu)ther colony of the same species not ver\" far 
distant, at a slightlv higher or lower elevation, may pi'esent very con- 
siderable variation in one or more of these characters. 
This variation or tendenc}" thereto is sometimes exhibited when a 
large number of individuals ai'e brought together from a pond or 
spring of large area where the volume of water is subject to seasonal 
mutations. Binney in illustrating Limnma palustris^ and certain other 
species of this genus has very properh^ given several figures. The 
same course is necessary in the Physas. 
In Plate XXII, which folloAvs, may be seen the figures of the types 
of several species, as given by Binney,'* which will facilitate compari- 
son with the figures in Plate XXIII and enable the student to draw his 
own deductions, though, as before stated, no species of PJiysa can be 
satisfactorih" illustrated by a single figure, however accurately di-awn. 
The following notes have reference to the figures on Plate XXIII: ^ 
No. 7. Upper part of basal whorl extremely tabulated; variation from P. hnrnerom? 
Kg. 8. Basal whorl flattened; apex depressed and short; the same features less pro- 
nounced exhibited in 9 and 10. 
No. 11. Apex short; basal whorl rounded. 
No. 12. Much like a lar^e ten; apertures patulous and reflected below. 
Nos. 21, 27, 28, and 49. Outer lip pinched above and sinuous. 
Nos. 28, 39, 46, and 47. Heavily calloused on body whorl, forming with tlie outer lip 
a nearly continuous rim. 
No. 33. Anterior portion of aperture or lip thrice repeated; see also fig. 4 in Plate \l. 
Through the courtesy of the Secretary of the Smithsonian Institution 
the author has been permitted to use the cuts in Plate XXII; he is also 
indebted to Prof. W. H. Dall and Mr. C. T. Simpson of the U. S. 
National Museum for kind attention to various matters on several 
occasions. 
^ Land and Fresh Water Shells of North America, Part II, p. 47, and elsewhere. 
^ Idem, p. 75, et seq. 
^The majority of the forms have been compared wdth material in the National 
Museum; those not referred to in the index or notes are left to the judgment of the 
reader. ]Many of the determinations may be regarded as arbitrary; criticism on this 
point is excusable. 
