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,}+ and Clathurella rugosa, Mighels.{ 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 
Fig. 42. 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. 
+ Hervier—Journ. de Conch., xliv., 1896 (1897), p. 147; xlv., 1897, p. 
110, pl. iii., fig. 3. 
t Langkavel—Donum Bismarckianum, 1871, p. 2, pl. i., fig. 5. 
