216 



Piiysa bullata. T. magna, ovato-ventricosa, tenuis, fragilis, 

 lucida, cornea: spira elevata, acuta; anfr. 6, ultimo inflato ; sutura 

 bene impressa : apertura late ovata, 4 long, testa? adequans ; labro 

 tenui, rufo submarginato ; columella valde flexuosa, callo induta. 



Long. 1 poll. ; lat. J$ ad \\ poll. Found in Oregon, by Dr. J. G. 

 Cooper. 



Distinguished by its large size, inflated form, and delicate structure ; 

 sometimes the form is somewhat cylindrical. It accords most nearly 

 with Haldeman's pi. 3, fig. 9, which was given him as P. Sayi Tap- 

 pan. It is much more delicate, and less polished than P. heterostro- 

 pha Say, and the aperture is less elongated. 



Pibtsa humerosa. T. subrhomboidea, solidula, polita, albida : 

 spira acuta ; anfrac. 5 tabulatis : apertura \ ad J- long, testae ade- 

 quans, postice rotundata ; labro expanso ; columella vix plicata, 

 callosa, fere perforata. 



Long. \ ad T r T poll; lat. § poll. Found by Dr. Thomas II. Webb, 

 and by W. P. Blake, in the Colorado Desert and at Pecos River. 



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

 foldless pillar clearly distinguish this species. It is like P. tabulata, 

 Gould, and the variety figured by Haldeman, as P. ancillaria, (fig. 7,) 

 which he regards as a monstrosity ; the deep suture and simple colu- 

 mella distinguish it from that species. 



Piiysa virgata. T. modica, solidula, glabra, elongato-ovata, 

 cinerea olivaceo longitudinaliter virgata : spira elevata, acuta ; anfrac. 

 4-5 bene* discretis : apertura lunata, ^ longitud. testa? adequans ; colu- 

 mella modice plicata, valde callosa; faucibus luteo-rufescentibus. 



Long. -| ; lat. ^ poll. River Gila, near San Diego. Dr. Thomas 

 H. Webb. 



Quite remarkable as being the only species yet known with varie- 

 gated coloration. The stripes are found on some part of every shell, 

 and many are prettily ornamented throughout. In size and propor- 

 tions it may be compared with P. microstoma, Hald. 



Plaxorbis ammon. T. magna, discoidea, subconica, subtiliter 

 striata; latere sinistra late et profunde concavo, anfractus 4 obtuse 

 carinatos exhibente ; latere dextro concavo, anfr. 2£ rotundatas 

 monstrante : apertura ovato-triangularis, interdum utroque valde 

 expansa. 



Axis, | ad 1 ; diam. 1 ad 1 poll. Cienaga Grande, the Colorado 

 Low Desert. T. H. Webb and W. P. Blake. 



Differing greatly in size and the development of the aperture, all 

 agree in the peculiar slope of the outer volution, giving them a coni- 

 cal or dome-shaped form when lying on the left side. Fully developed 

 specimens are much like P. corpulentus, Say, but the shape of the. 

 volutions and aperture differ widely. 



