PILSBRY: non-marine MOLLUSCA of PATAGONIA. 575 



Development. — The neanic stage in this species is like that of P. 

 bisiiiimtus. The ephebic stage is described above. A single gerontic 

 individual before me has built the median region of the outer lip forward, 

 obliterating the median notch of the lip (PI. XLI, figs. 2, 2a). 



This remarkable form is the most advanced of the bisiiinatiis phylum 

 known. It resembles P. bisiuiiatns in contour, and neanic shells of the 

 two species are distinguishable only by the bands of the latter, when these 

 are developed. The ephebic stage differs widely by the strong expansion 

 of the outer lip, its thickened and thrice notched face. 



What relation P. bisiniiatiis and P. sykesi bear towards P. petitianus 

 (d'Orb.) of the Parana River is unknown, pending the discovery of the adult 

 stage of the latter, the specimen described and figured by d'Orbigny being 

 supposed to be immature. It is likely that petitianus will prove to be 

 different from either of the other species. 



PoTAMOLiTHUs PETITIANUS (d'Orbigny). 



Paludestrina petitiana d'Orb., Voyage dans I'Amerique Meridionale, Mol- 

 lusques, p. 487, pi. 75, f. 19-21 (1839). 



Shell short, ovate-inflated, thin, smooth, not umbilicate ; spire short, 

 eroded, with obtuse summit, composed of five convex, narrow whorls, 

 parted by a suture which is not very deep. Aperture oval with simple 

 margins. Color green, with three reddish bands, one on the convexity of 

 the spire, the others at the suture and anterior. Alt. 4, diam. 3 mm. 

 {d'Orbigny). 



Parana River at San Pedro, Argentina, collector unknown. 



This may be the neanic stage of a species allied to P. bisinnattts ; but 

 in this group of forms [bisimiatns and sykesi) the neanic stage shows no 

 specific differentiation, the specific characters appearing only in the final 

 stage of development. Until the adult form of P. petitianus is collected 

 at the type locality, San Pedro on the Parana, no good purpose will be 

 served by uniting either of the other species io petitianus as its hypotheti- 

 cal adult. There remains also the possibility that P. petitianus is a per- 

 manently undeveloped form, not passing beyond the neanic stage of the 

 bisinuate species, and therefore falling more properly in the lapiduni 

 group. Compare also P. paranensis, p. 589. 



