REPORT ON THE GASTEROPODA. 103 



6. Basilissa costulata, Watson (PI. VII. fig. 11). 



Basilissa costulata, Watson, Prelim. Report, pt. 3, Journ. Linn. Soc. Lond., 1878, vol. xiv. p. 600. 



„ Dall, " Blake " 4 Moll., Bull. Mus. Comp. Zool, Camb., U.S.A., 1881, vol. ix. p. 48. 



Station 24. March 25, 1873. Lat, 18° 38' 30" W., long. 65° 5' 30" W. Off Culebra 

 Island, West Indies. 390 fathoms. Pteropod ooze. 



Habitat.— Gulf of Mexico, 15 to 640 fathoms (Dall), Stations 7 (18G8 or 1869), 

 16, 17, 17a (1870) of the "Porcupine" Expedition (Jeffr.) 



Shell. — Small, depressedly conoidal, sharply angulated, flattish on the base, sharply 

 and deeply umbilicated, a little porcellanous, flexuously ribbed. Sculpture: The whole 

 surface is covered with longitudinal flexuous ribs, which are narrow, sharp, and uniform ; 

 each of these is about 0-002 in. broad, and they are parted by intervals, which at the suture 

 are twice and at the periphery thrice as broad as the ribs. There are about forty of these 

 on the last whorl. They are crossed by spirals, half the breadth of the ribs, running across 

 the intercostal spaces and forming knobs on the ribs. Of these spirals there are about 

 seventeen on the last whorl, much closer set and less uniform than the ribs; in particular 

 the carinal spiral, which is very sharp, and the fourth and seventh above it, are stronger 

 than the others ; the last mentioned of these is especially so on the earliest whorls. On 

 the base the longitudinals, though continued even into the umbilicus, become much less 

 prominent and are no longer nodose ; and the spirals, of which there are about fifteen, are 

 closer set, broader and flatter, except the first three below the carina, which are sharp and 

 narrow. The whole base is pit-marked from the spiral interstitial furrows being cut up 

 by the longitudinals. Colour dead white (on the base a little glossy) on the thin porcel- 

 lanous surface, through which the nacreous layer behind gleams. Spire raised, with a 

 very slightly concave outline. Apex, which consists of the minute smooth embryonic whorl 

 and a half, is itself a little flattened, but rises sharply above the sculptured surface of the 

 succeeding whorls. Whorls 5^ of slow and regular increase till the last, which begins to 

 enlarge rapidly ; they are slightly rounded above, very sharply angulated at the keel, and 

 flattened on the base, with a bluntly angulated umbilical edge. Suture distinct, slightly 

 impressed. Mouth perpendicular, square. Outer Up sharp, thickened by a slight internal 

 remote callus, not patulous, not descending, advancing at its junction with the body-whorl 

 and then slightly retreating so as to form the very shallow open sinus ; right-angled at 

 the periphery, flat on the base, where it retreats so as to form two rounded sinuses, making 

 with the pillar an angle that is scarcely obtuse. Pillar-lip is straight, a little thickened 

 and reverted, so as to leave a slight groove behind it. It advances on the body- whorl, 

 then retreats so as to form a feeble sinus, bending at the same time shortly but sharply to 

 the right into the umbilicus and then advancing straight forward, but a little toward the 

 left, to its junction with the outer lip at the base. Umbilicus funnel-shaped, open- 



