Thalassohelix. ] GASTROPODA. 635 



fine growth-lines and inconspicuous dense microscopic spiral strise, 

 which, however, are only visible with a magnifying-power of about 

 50 diameters. Colour horny -brown, the last whorl usually of a lighter 

 colour, green-corneous. Epidermis thin, faintly shining. Spire de- 

 pressed, slightly convex, not quite half the height of the aperture. 

 Protoconch flattish, of li whorls, finely radially and microscopically 

 spirally striate. Whorls 4|, slowly increasing, lightly convex, the last 

 not- descending in front, obtusely angled at the shoulder. Suture deep. 

 Aperture oblique, round-lunar. Peristome thin and simple, advancing 

 above. Columetta short, oblique, arcuate. Inner lip very little re- 

 flected above. Umbilicus broad and deep, about one-quarter of the 

 greatest diameter. 



Diameter Maj.. 9-5 mm. ; min., 8-25 mm. : height, 4mm. to 

 4J mm. 



Animal. Top of the head yellow ; peduncles and a line on each 

 side of the head black ; rest of the body pale grey spotted with dark 

 grey, the spots often collected into groups ; a regular line of dark-grey 

 spots runs along each side, and they meet on the tail just over the 

 mucous pore ; below this line the side of the foot is marked with 

 oblique dark stripes ; sole white, the margin with grey spots. 



Dentition. Hutton, T.N.Z.I., xvi, 166, pi. 9, f. I ; pi. 11, f. R : 

 Man. Conch. (2), ix, pi. 2, f. 4, 5. 



Jaw slightly arcuated, with numerous narrow vertical plaits. 



Radula (Plate 1, fig. 10) having the formula 24 + 1 + 24 ; laterals 

 8 or 9. Central tooth tricuspid, the median cusp reaching the pos- 

 terior margin ; all cusps with moderate cutting-points. Laterals 

 similar to the central, the transition teeth bicuspid. Marginals longer 

 than broad, with a broad bidentate point. 



Type in the Canterbury Museum. Christchurch. 



Hob. Greymouth, type (R. Helms) ; Capleston and Reefton 

 (Cavell). 



Genus 4. ALLODISCUS, Pilsbry, 1892. 



Allodiscus, Pilsbry, '' Nautilus," vi, Sept. 1892, 56. Type : Helix dimorpha, 

 Pfeiffer. Psyra, Hutton, T.N.Z.I., xvi, 1884, 201 ; not of Stal, 1876, a 

 genus of Orthoptera. Allodiscus, Pils., Man. Conch. (2), viii, 66; ix, 14. 



Animal with the mantle subcentral ; tail truncated, with a mucous 

 gland below a papilla ; eye-peduncles thick, approximated at their 

 bases. 



Jaw slightly arcuate, not tapering toward the ends, flatly ribbed 

 fir plaited. 



Dentition : Central tooth with tricuspid reflection, mesocone long. 

 Lateral teeth bicuspid, the entocone being suppressed, or tricuspid. 

 Marginal teeth broad, with 3 to 5 cutting-points. 



Shell thin, orbicular, and depressed, with low or flat spire, rounded 

 periphery, and narrow subimperforate or quite closed umbilicus ; 

 surface radially rib-striated, minutely reticulated, not hairy, the 



