AND ALLIED FORMS. 49 



projection of the outer lip, which gives it a character somewhat 

 like that seen in Campeloma and Ehurna. 



In company with the Tryonia; Mr. Blake found a small cancel- 

 lated shell which has been described as Melania exigua by Conrad 

 and as Amnicola jwotea by Gould.^ In view of the character of 

 the surface, I think it scarcely possible that this species can 

 belong to the Hydrobiina^. It will, perhaps, be found to be allied 

 to Bittium. The occurrence of this marine or brackish-water 

 genus in the Desert would not be surprising, since Gnathodon 

 was found in the same basin at a point somewhat nearer the Gulf 



It has, unfortunately, been only possible to describe this genus 

 in a very imperfect manner, the characters of the shell alone 

 being given. I have dedicated it to Mr. Geo. W. Tryon, a 

 well-known conchologist of Philadelphia, to whom we have been 

 indebted for the loan of many interesting specimens of Amnicolse, 

 etc. 



POTAMOPYRGUS,- Stm. 

 Potamopyrgus, Stimpsox, Am. Journ. of Conch., I, (1865) 53. 



Shell ovate-conic, imperforate ; apex acute ; whorls coronated 

 with spines ; outer whorl nearly two-thirds the length of the 

 shell ; aperture ovate ; outer lip acute. Operculum corneous. 

 Rostrum of moderate length. Tentacles very long, slender, 

 tapering, and pointed. Eyes on very prominent tubercles. Foot 

 rather short, broadest in front and strongly auriculated. 



Lingual dentition of type : Rhachidian tooth trapezoidal ; in- 

 ferior margin nearly straight, faintly trilobate ; basal teeth minute 

 and close to the lateral margins. Intermediate tooth with the 

 peduncle veiy long, three times as long as the body and con- 

 stricted at its juncture therewith ; body subrhomboidal and ex- 

 cavated in the middle ; cusp with numerous equal teeth. Lateral 

 teeth constricted near the summit, and with the dorsal or outer 

 margin of the shank reflexed or thickened ; outer lateral with a 

 broad summit shaped like a chopping-knife. Formula of the 

 denticles: ^-11-15-20. 



Station, fresh water. 



Distribution, New Zealand. 



' Pacific R. R. Reports, Vol. V, p. 332, pi. xi, fig. 6-9. 

 2 rrora/xoc, fluvius ; trv^yo;, turris. 

 June, 1865. 



