198 GASTROPODA. [Pectinibranchia. 



Subgen. EURYTROPIS, Kob. & Mlldff., 1898. 



Shell conically umbilicated, with very prominent umbilical keel, 

 forming a wide semicircle around the umbilicus ; mostly brightly 

 coloured, streaked or banded. 



1. Omphalotropis vestita, L. Pfeiffer, 1855. 



Hydrocena (Omphalotropis) vestita, Pfr., P.Z.S., 1855, 106. Omphalotropis 

 vestita, Pfr., Monogr. Pneum. viv., ii. 166 ; Hedley and Suter, P.L.S. 

 N.S.W. (2), vii, 1892, 622 ; Suter, J. de Conch., xli, 225 ; Kobelt, Jahrb. 

 Nassau. Ver. f. Nat., lix, 84. 



Shell perforated, oblongly conical, thin, striated and spirally 

 closely ridged, covered with a somewhat fuscous epidermis. Spire 

 pyramidical, rather acute. Whorls 6, flat, the last armed below the 

 middle with an acute keel, and about the perforation with a second. 

 Aperture hardly oblique, subangularly oval. Peristome simple, straight, 

 margins converging. Operculum fuscous. 



Diameter, 3 mm. ; height, 5 mm. ; length of aperture, 2 mm. 



Type in the British Museum. 



Hab. New Zealand (ex coll. Cumingiana). 



Remark. This species has never been found again, and it is doubt- 

 ful whether the type was really found in New Zealand. 



Fam. RISSOID^], Gray. 



Animal with a more or less elongated muzzle ; elongated cylindrical 

 tentacles, with the eyes on swellings at their outer bases ; verge ex- 

 serted, situated at a considerable distance behind the right tentacle ; 

 gills both pallial, the right one largest, rather short and broad, com- 

 posed of few lamina, which are much broader than high ; epipodial 

 filaments present ; 1 or 2 pallial tentacles ; foot oblong, punctate in 

 front, rounded or pointed behind. Operculigerous lobe well developed. 

 Jaws oval, reticulated. Dentition 2+1+1+1 + 2. 



Shell small, usually turbiuate or elongate, often more or less um- 

 bilicated ; peristome continuous, more or less rounded, never truly 

 channelled in front ; operculum corneous, paucispiral. 



The Rissoidce are small, often minute, phytophagous animals of 

 marine habitat, found in all parts of the world, frequenting alga3, 

 zostera, &c., from which they often suspend themselves by a mucous 

 filament. 



KEY TO GENERA. 



A. Aperture entire, oval, peristome thick, simple or re- 



flected ; operculum corneous, paucispiral . . . . RISSOA. 



B. Aperture semilunar, mostly a little reflected, anteriorly 



effuse or faintly channelled ; operculum corneous, 



\vi1li a elaviform process on the internal face . . RISSOINA. 



C. Aperture separated from the body-whorl . . AMPHITHALAMUS. 



D. Shell scahiiiloi in, with a carinated shoulder .. .. ANABATHRON. 



E. Shell depi-essed, orbicular, umbilicated ; opcrculum with 



a large process, vertical to the nucleus . . SKENELLA. 



