H. Doc. 129. 23 



The following are descriptions of species believed to be new. Two 

 or three species are left undetermined in consequence of the specimens 

 being imperfect, or the means, for deciding being deficient : 



FOSSIL FRESH-WATER SHELLS FROM THE COLORADO 



DESERT. 



PHYSA HUMEROSA, Gould. 



T. solidula, subrhomboidea, polita; spira acuta, anfractibus 5 tabu- 

 latis; apertura \ ad f long, test adequans, portice obtusa; labro 

 expanse, columella vix plicata. PI. ,fig. . 



Shell rather large and solid, subrhomboidal, polished ; all the 

 specimens seen, (quite numerous, and apparently not weathered,} 

 porcelain white. Whorls five, each with a broad square shoulder, 

 and forming an elevated, acute spire, aperture usually about two- 

 thirds the length of the shell, sometimes but little over one-half ; pos- 

 terior angle obtusely rounded ; outer lip slightly flaring ; when 

 viewed in profile it is slightly advanced posteriorly so as to form a 

 recess at the junction ; pillar without any conspicuous fold, thickly 

 covered with enamel, broadly rounded and expanded at the base. 

 Umbilical region nearly perforate. 



Length on an ordinary specimen, half an inch ; breath, three-eighths 

 of an inch ; length of a variety, seven-tenths of an inch. 



Found in the Colorado desert, by W. P. Blake, also at Pecos River, 

 by Dr. T. H. Webb. 



The broadly tabulated whorls, with the acute, elevated spire and 

 folded pillar,, clearly distinguish this species. P. tabulata, Gould, 

 from New Zealand, is similar, as well as some varieties of P. ancilla- 

 ria, Say, as figured by Haldeman, especially his figure 7, wbich he 

 designates as a monstrosity ; but the spire is more elevated, and the 

 deep suture always renders the whorls distinct, and the absence of a 

 columellar fold is a still further distinctive mark. It would be diffi- 

 cult to distinguish the young by themselves from several other species, 

 though they begin to show the angularity of the whorls quite early. 



PLANORBIS AMMON, Gould. 



T. magna, discoidea, subconica, subtiliter, striata ; latere sinistro- 

 late et profunde, concaio, anfractus quatuor obtuse angulatos exhibente ; 

 latere dextro excavate, anfractus duos cum dimidis ostendente ; apertu- 

 ra ovata-triangularis, interdum utioque valde expansa. PI. , fig. . 



Shell discoidal, attaining a large size, delicately striated, of a wa- 

 tery white color ; when laid upon the right side, the shell has a 

 remarkably conical or dome-shaped aspect, the extreme periphery 

 being in a plane with the right side, or base, from which the whorl 

 gradually slopes upwards, terminating at the summit in a broad and 

 deep concavity, exhibiting four well marked, obtusely angulated 

 whorls ; the right side is decidedly concave, exhibiting two and a 

 half or three well rounded volutions. Aperture ovate-triangular, in 



