THE MOLLUSCA HEDLEY. 



473 



characters have little absolute systematic value in this group, 

 and their relative value remains to be determined."* 



Even should little weight attach to the nuclear distinction of 

 Thetidos, the aperture, so curiously imitating Sistrum or Pupa, 

 may separate it from its kindred, only excepting Clathurella idio- 

 morpha, Hervier,f and Clathurella rugosa, Mighels.l As those 

 authors paid no special attention to the protoconch, I am unable 

 to decide whether they should also enter my genus. 



I have no information relative to the presence or absence of 

 the operculum, since to obtain such would entail the destruction 

 of the only shell. It may be that in this family the thickening 

 of the lip, followed by the development of the labial teeth, and 

 consequent narrowing of the aperture has accompanied the de- 

 generation of the operculum. The safety of the animal being thus 

 secured by the exchange of one defence for another. 



THETIDOS MORSURA, sp. nov. 

 (Fig. 42). 



Shell stout and strongly 

 built, briefly conical, a little 

 turreted, anteriorly narrowed 

 suddenly with a short straight 

 and truncate canal. Whorls 

 five, exclusive of the proto- 

 conch. Colour dead white, 

 except the two uppermost 

 whorls and the protoconch, 

 which are pale fawn. Sculp- 

 ture the last whorl has ten 

 thick and prominent ribs, 

 round at their base and 

 summit, their own width 

 apart, shouldered posteriorly 

 and abruptly terminating 

 anteriorly at the basal con- 

 striction. On each succeeding whorl the ribs alternate with those 

 beneath. The revolving sculpture consists, on the last whorl, of 

 eight, strong, elevated, equidistant, narrow spiral cords which 

 over-ride the ribs, and five such which encircle the base, where 

 vestigial ribs tend to dissect them into nodules ; on the penulti- 

 mate whorl there are four to five cords visible. Protoconch tilted, 

 two-whorled, and spirally grooved. Aperture narrow ; columella 



* Dall Loc. cit., p. 75. 



t Hcrvier Journ. de Conch., xliv., 1896 (1897), p. 147 ; xlv., 1897, p. 

 110, pi. iii., fig. 3. 

 J Langkavel Donuin Bismarckianum, 1871, p. 2, pi. i., fig. 5. 



Fig. 42. 



