Of the desert region outside the California portion little is known, and the California area, so far as Mollusca is concerned, has only been partially explored. FOSSIL SHELLS OF THE COLORADO DESERT T— STEARNS. 273 circumstances under which the examples from this remote locality . was detected. The evidence for lacustrine or lagoonal conditions resulting from the overflow of the Colorado River in the past is confirmed by the perpen-.
The forms in question were collected by the writer in 1882, in the immediate vicinity of several stations along the line of the Southern Pacific Railroad in the Colorado Desert of California. The length of the opening and its position in relation to the axis of the shell varies in different examples. In all of the following examples, the spiral sculpture is seen and more or less noticeable.
In fig. 13 a repetition of the slimmer aspect is seen, and in fig. 18 the maximum is shown. With the exception of the New Mexico and Durango localities, all others are represented in the U. S. NationalMuseum collection. In connection with the foregoing, the suggestion arises that of the many fountains now.
We have no certainty about the proportions of the mantle and its size in relation to the size of the mouth and other characters.
OTHER SPECIES OF THE AMNICOLID^
It most probably varies in density one time compared with another according to the proportions of conchiolin or lime; with the first. It may be that the edge of the mantle is thin and plain; that a series of pores occurs parallel to, but not quite at, the edge of the mantle; that these pores are almost equally spaced with respect to distance from each other. In this case any differentiation in the size of the lira—that is, coarseness or fineness—will result from a lack of uniformity in the size of the pores, and irregularity or differences in distance from each other may be attributed to the partial or absolute crowding of some of them.
Angle or shoulder formation, in some cases indicating absolute deformation, observed in asymmetry and hump, is thought to be due to hypertrophy of the visceral mass, especially the liver, and to entanglement at a certain time during the growth period in the plant material in the middle of which they live, and the flare of the mouth, as seen in fig. The following forms collected by the Death Vallev expedition in 1891 were first made known in the report of said expedition published in 1893.'^ As the report has long been out of print and many persons engaged in the study of the mollusca interested, having never seen the descriptions or figures, it is advisable to republish them in this connection, although the shells have not yet been traced in that part of the Great Desert to which this article mainly refers. Aperture ovate, about half the length of the entire shell, obtusely angular above; the inner lip is either free from the preceding whorl or in contact only at the upper part.
In the same spring, Merriam located numerous living examples of the long sought-after Tryonla dathrata Stimpson, formerly known. It is clear that the term "Colorado Desert," as used in connection with the Blake-Carlton Shales, included a much larger portion of the Great Basin than is now understood when called the "Colorado Desert." become Attention is drawn again to the Hgure "139, Tryonla dathrata^'''' as given in Binne}^^ which Dr.
THE PHYSAS
287 proper Ligurian Researches of the last author on Hydrobmaj/. 288 PROCEEDINGS OF THE NATIONAL MUSEUM. the maximum of development in this direction is farther south, as shown by the fine, large, very smooth shell of the western Mexican species P. Aplexa) aurmitia Carpenter, from Mazatlan. That several species of Limncea occur in many localities in the northern Great Basinton, at various altitudes from 1,300 to 4,0()0 feet^. and up, and east into Arizona at Tucson, elevation 2,300 feet, to the highest elevation of Walker Lake, San Francisco Mountain. elevation, at Daggett, on the Mojave River, in the Mojave Desert, 2,000 feet, points to the thermal factor of the lower levels in this region as the unfavorable feature which excludes the Limncean from the desert. The Lower California and other species of the genus occur in the south not far from the coast, at lower levels, within reach of the cold winds and fogs of the Pacific.
To facilitate comparison, in Plate IV. We present several figures from Binney* of described species from regions both north and south of the desert. With the previously mentioned distributive agents in mind, it would be good to look at these forms and note the places where they were discovered. jyrlna, which he suggests is a variety of P. heterostropJia, at the. The same author notes of jP. heterostropha that it has not been found in the Bonneville or Lahontan lake beds, but is abundant as a semi-fossil on the surface of the Sevier Desert.
VernonBailey among a moss "irrigation ditch" in Phoenix, Arizona, and from the same site in Magdalena, northwestern Mexico. Some strong-shouldered specimens in the subfossil state from the Colorado Desert may be a variant of this species, which is. ^ described from samples collected by Dr. Webb in Arizona's Gila River; it also occurs near San Diego and Los Angeles, where I have collected numerous specimens.
These give a good idea of the differentiation of Physa humerosa from that locality, a species quite persistent in its main characteristics and first described from Colorado. Additional examples may reveal it to be a long-pointed variety of ahUa, found in Lower California, or a slender aspect of niiens, a Mexican species. .. 290 ACTIVITIES OF THE NATIONAL MUSEUM. vol.xxiv. . a first series, which showed the extremes and other aspects of variation more clearly, was unfortunately misplaced. The plate doesn't give a good idea of the size as a whole; the examples selected are simply intended to illustrate variation; they are predominantly)!}^ below average.
SCULPTURE AND SALINITY
290 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxiv. . serios first, which displayed the extremes and other aspects of the variation more prominently, as they were unfortunately badly placed/. Physa carltoni Lea, from near Antioch, at the junction of the Sacramento and San Joaquin rivers, is sometimes damaged. All the various sculptural aspects above described also occur in the Limnceidce^ of which the following species may be named: L. caj)erata^ L. cataseopiiim^ L. colinneUa, L. decoUata, L. emarginata,L. elodes, L.lanceata, L. lepida.. L. umhrosa^ and of the innumerable varieties and races above mentioned, and Call's fossil L. honnevillensis, from the quaternary of the Great Basin, and its lifeZ. {Radix) ampl., var. utahensl.s from Utah Lake.
It will be observed on reference to the descriptions of the many .. species of Physa here quoted, that no mention is made of salinity in connection with the waters in which the shells were found; the same remark also applies to the various species of Liiiiiuea mentioned above. Themalleered aspect, etc., in Z. aatpla^ and these in many and various aspects of differentiation in Limnceaeinarglnata^ which may ])een in the National Museum. In the large series from Eagle Lake' in central Minnesota, where neither saline nor thermal conditions are to be considered, nearly all, if not all, the sculptural characters occurring in the numerous.
The partly or wholly ground surface which is so often met with in the Limnceids, regardless of the altitude or the salinity of the water, and less frequently in the Physas^ is explained by the character of the lake or pond-bed in which these dinted forms occur. The character of the bottom, even in a pond of limited size, often exhibits very considerable differences in the matter of compactness or density; alluvial .. mud, clayey mud, clay or sand, with teeth or coarse gravel mixed with fragments of aquatic plants and plant stems in various proportions. The habits of these molluscs include, if not properly speaking- digging, rolling or submerging, and moving as they do with somewhat of a rotary motion, this, combined with the moderate impact of the surrounding mattin-. contribute to the production of the milled or colored surface, which frequently exhibits a somewhat spiral arrangement.
SCULPTURE AND THERMAL WATERS
SIZE AS RELATED TO HYPSOMETRIC TEMPERATURES
While elevation, which in this connection is equivalent to reduced temperature, has apparently no sculptural influence, it appears to be closely related to size in Physas. This aspect of depletion, called dwarfism, is shown in Call's (Plate XIV) comparative measurements^ of Physa mnpuUacea from Little Gull Lake in the Mono Basin of California, elevation 7,000 to 7,500 feet, and Church Lake near Salt Lake City, Utah, elevation. As regards the relation between exhaustion and salinity, CalTs table (XI), giving the measurements of thirteen cases of Physa gyriiia.
While numerous examples of 1\ gyrlna were collected by Dr. C. Hart Merriam in Bennett Springs, MeadowValley, Nev^ada, elevation 6.0<)0 feet, and the large number (492) of P. Comparison of forms from high elevations with those of stations nearer the level of the sea will be more satisfactory than that quoted above, where both places are hill stations.
VARIATION IN FORM
It is rather absurd, considering the present state of our knowledge of the sensitivity of the molluscs of this family to environmental influences, to investigate any species of Physa by a single ligure. Although the shells of a colony of a given species located in a spring or pool of limited area may be nearly or completely uniform in size, shape, texture, and color, the colony of the same species is not far away. At slightly higher or lower altitudes there may be very significant variation in one or more of these characters. In Plate XXII, which follows Avs, the figures of the types of several species may appear, as given by Binney. facilitates comparison with the figures in Plate XXIII and enables the student to draw it.
The majority of the schemes are compared with broadside material in the National Museum; those not referred to in the index or notes are left to the judgment of the reader.
THE PLANORBES
BIBLIOGRAPHY
EXPLANATION OF PLATES
