576 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



POTAMOLITHUS BISINUATUS Pilsbiy. 

 (Plate XLI, Figs. 6-7*.) 



Potamolithus bisimiatus Pilsbry, Nautilus, X, p. 88, Dec., 1896. 



The shell is imperforate, globose-conic, moderately solid, green or pale 

 yellowish-green, sometimes one-colored, but usually begirt with three nar- 

 row reddish-brown bands, one bordering the suture, another above the 

 periphery, and on the penultimate whorl visible above the suture, and the 

 third band below the periphery. The nearly smooth surface is weakly 

 marked with lines of growth. The spire is conic and rather high, trun- 

 cated at the summit in all adult shells seen, by the erosion of the early 

 whorls, about y/ 2 remaining. These are strongly convex, the last one 

 globose, without keels or angles of any kind, and with no expansion or 

 varix behind the outer lip. There is a distinct and concave but quite 

 small columellar area. The aperture is moderately oblique, round-ovate, 

 white or brownish inside. Its posterior angle is more or less filled with 

 a callous deposit. The peristome is edged with a black line. The thin 

 outer lip has a deep rounded sinus near its posterior insertion, and there 

 is a second sinus, wider and not so deep, at the base, the lip projecting 

 as a broad truncated lobe between the two embayments. The columella 

 is concave, narrowly calloused and the parietal callus is rather thick. 



Length 5, diam. 3.9, length of aperture 2.8 mm. 

 " 4.8 " 3.3 " " 2.9 " 



Uruguay River at Paysandu. Types collected by Dr. W. H. Rush, U.S.N., 

 May 7, 1892. 



Development.--^^ shell is of the ordinary simple Naticoid shape 

 throughout the neanic stage, differing from P. lapidimi only in having a 

 longer spire. The peculiar Pleurotomoid sinuosities of the peristome 

 have their origin and development wholly in the ephebic stage. In this 

 respect, P. bisimiatus is like Pachycheilus dalli Pils., and differs widely 

 from Gyrotoma and Pleurotoma, in which the anal notch appears very early. 



This species is related to P. sykesi, from which, however, it differs totally 

 in characters of the ephebic stage. 



