Thalassohelix. ] GASTROPODA. 633 



thin, horny, easily peeling off. not shining. Spire convex, conoidal, 

 scarcely half the height of the aperture. I'm'acnnch flat, of If smooth 

 whorls. Whorls 5|. rather convex, regularly increasing, the last not 

 descending, with rounded periphery ; base convex. Suture deep. 

 Aperture oblique, rotundly lunar. Perisloinc simple, thin and sharp, 

 regularly arched, the upper part aivuatdy protracted below the inser- 

 tion. Columella short, somewhat oblique, arcuate. Inner lip slightly 

 callous, shortly reflexed. spreading as a very thin white glaze over the 

 convex parietal wall. Umbilicus moderate, somewhat perspective, 

 deep, its diameter about one-sixth of the greatest diameter of the 

 shell. 



Diameter Maj., 8 mm. : min.. 7 mm. : height, 4 mm. 



Dentition. Hutton. T.N.Z.I., xvi, 165. pi. 9.Y T ; Man. Conch. (2). 

 ix, pi. 3, f. 28. 



Jaw arched, not tapering, with about 34 flat plaits. 



Radnlit has the formula 24 + 1 + 24 ; laterals about 9. The 

 central tooth rather wedge-shaped, the reflexed portion triangular. 

 with a small point. Laterals rather oblique, broader than the central, 

 the reflexed portion bicuspid with a long point on the inner cusp ; 

 the}" pass gradually into the marginals, which are rather broader than 

 long, with a long cutting-point and a small one on each side of it. 

 (Hutton.) 



Type in the U.S. Nat. Museum, Washington. 



Halt. North Island : A crater near Taiaimi, type (Pickering) ; 

 Parua Bay, Whangarei ; Hillyer's Creek, near Auckland ; Mount 

 Wellington (Musson) ; Waiheke Island (H. S.) ; Waiwera ; Swanson ; 

 Waitakerei Range (H. S.) ; Hnnua Range : Tarukenga (Major Broun) ; 

 Tuakau (S. Bull) ; Ngaputahi. Tuhoe-land (A. Hamilton) ; Rua- 

 tahuna, Tuhoe-land (E. Best). 



Remark. Erroneously reported from Australia. 



Genus 3. GERONTIA. Hutton. 1883. 



Gerontia, Hutt., T.N.Z.I., xv, 1882 (1883), 135. Type: G. pantherina, 

 Hutton. Gerentia (typog. err.), in N.Z.J.S., i. 476. Gerontia, Hutt, : 

 Pilsbry, Man. Conch. (2), ix, 14. 



Animal heliciform ; mantle rather posterior, included : tail acute, 

 with a mucous pore, but no papilla. 



Jaw vertically plaited. 



Central tooth of radula tricuspid. cusps with moderate cutting- 

 points. Laterals similar, but the ectocones larger than the entocones ; 

 transition teeth bicuspid by fusion of entocone with mesocone. 

 Marginals with a broad bifid cusp. 



Shell depressed and openly umbilicated, having a contour of 

 Patula ; thin, rather fragile, the surface delicately sculptured with fine 

 cuticular riblets ; apical whorl minutely granular, or showing a few 

 weak spirals, having a minute perforation at the tip. 



