OCR Text |
Show 374 MR. R. B.WATSON ON MADEIRAN MOLLUSKS. [Mar. 18, strong relatively to thickness of shell, is cut off by a nick from previous whorl, is very sinuous relatively to plane of spire, is very faintly crossed by spiral threads. Spiral threads on body-whorl 7-9, transparent, prominent. The first, nearest suture, always projects less than the others, and often disappears, as does also the second occasionally. The fifth is the prolongation of the suture. The seventh and eighth (sometimes it is the sixth and seventh, rarely the eighth and ninth) are parted by an interval deeper and wider than any of the others ; hence a hunch on the base. The ninth merely encircles the pillar, and lies close to the outer left margin of the mouth. On the third and on lower part of the second whorls there are four of these threads; in the second whorl they are abruptly cut off; and above this, on the embryonic shell, they are replaced by six microscopic hair-like spirals. The interstices of the spiral threads are 3 to 4 times the breadth of the threads, and are delicately but very closely tooled with excessively minute undulated spiral scratches, of which about 4 go to y*-y*Tcj in. These cause the frosted appearance of the shell. When the longitudinal ribs are present, the threads in crossing them form faint knobs, and the whorl is cut into long narrow meshes. Colour pure white, transparent on the threads, frosted in the intervals. Spire rises in steps, is short, blunt, and truncated, the tip being turned in. Whorls 4-5, well-rounded, with a sloping shoulder below the suture, of very regular increase. Suture deep and very straight. Mouth very open, large for size of shell, very triangular in consequence of the extreme straightness of the line across the body and from tbe flattening on the base. Lip thick, with a sharp projecting flange on the inward side. At the outer lower corner it advances so much in front of the plane of the mouth as almost to form a sinus and a channel. Inner lip projected (rather than reflected) on the pillar, so as to form a distinct umbilical groove or chink. Across the body it almost dies away, but reaches the outer lip, which throws out a slight pad to meet it. Sh. M. 5. 4. 3. 2. 1. L *068 *027 -034 *016 *009 *006 '002 B *043 *025 *033 *037 *025 *015 -007 Hab. Gorgulho, shore; Monte de Piedade (Canigal), 15-35 fathoms; Ponta de Sao Lourenco, 25-45 fathoms; Funchal Bay, 50 fathoms ; Porto Santo, 50 fathoms. This species somewhat resembles R. subcarinata and R. aurantiaca. It differs from both in its broader form, whorls rising in steps, large spoon-like and triangular mouth, deeper suture, fewer and more rounded whorls, fewer spiral threads, the scrobicular parting of two of these on the base, the abrupt apex with in-turned tip, and its pure white colour. |