BY W. F. TETTERD. 71 



impossible and certainly unnecessary to enumerate all the 

 modifications. In many localities the whorls are more 

 or less sharply carinated, with sometimes the additional 

 ornamentation of a line of interrupted pointed spines, but 

 plain, carinated and spinose specimens are often found livinpj 

 in the same pool. The same pecularity has been noticed in 

 one or two of the New Zealand forms of the genus. 



In clear running streams the shells are often sub- 

 stranslucent and of a pale yellowish horn colour, but in quiet 

 still water they are usually coated with a thick covering of 

 decaying vegetable matter, generally of a rusty brown colour ; 

 often a closely packed mass of Diafomacece covers not only 

 the shell but also the operculum. In a small variety, collected 

 •at Deep Creek near the Duck Eiver, the penultimate whorl is 

 abnormally developed and the aperture constricted ; it is 

 possible that some conchologists may consider this and others 

 worthy of enumeration as varieties. Its nearest congener in 

 New Zealand is P. Antipodum, Gray ; it is also a variable shell, 

 extending in range throughout the whole of that colony ; it 

 is also found in brackish as well as fresh water. The teeth 

 agree in form with Dr. Stimpson's diagnosis, but the number 

 of denticles on the laterals present some modification ; this I 

 do not think of great importance. 



POTAMOPYRGTJS "WOODSII. H. sp. 



Plate I. Fig. 12. 



Shell, small, turbinately conical, thin, brown-horn colour, 

 covered with a thin epidennis, marked with irregular very 

 fine lines of growth. IVhorls 5, very convex, suture impressed, 

 spire short, apex obtuse, rounded. Body whorl large, inflated. 

 Aperture somewhat large and full, ovate, nearly one-third 

 length of shell. Peristome continuous, inner margin free, 

 forming an indentation behind, labrum thin. Operculum, 

 yellowish horn, thinpancispiral, with an internal submarginal 

 elevation. Animal, with long tapering tentaculce and project- 

 ing muzzle, coloured dark lead-grey, foot of moderate width, 

 white. Dentition, median both with the base broadly and 

 roundly expanded, pointed on either side, first lateral chib 

 shaped, but little l)ent. Second much curved and broad. The 

 third is much roiuuled aliovc, and the constriction at its 

 junction with the shank is deep, so that it forms a prominent, 



rounded, and curved tooth. 



f) 

 Formula of denticles, ^ ' ^ 11 ; 20 to 23. Number of 

 transverse rows, nearly 70. 



LaKjlh, 2>\\ hreculth, 2^ mill. 



