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Show 1878.] ' LIGHTNING'AND ' PORCUPINE' EXPEDITIONS. 413 the internal apparatus is different, and the beak is prominent Atretia and incurved in Dimerella. The septum in Atretia is short, gnomon-shaped, and central; in Dimerella it extends from below the beak to the front of the lower or smaller valve. The genus Mannia of Dewalque (Soc. Malac. Belg., 1874), from the " sable noir," or Upper Miocene of the Antwerp Crag, is also a Rhynchonellidan ; hut the internal apparatus is likewise different from that of Atretia. Each of the curved processes in the lower or smaller valve of Atretia gnomon has the outer end trifid, or is divided into three symmetrical digitate points. Genus II. RHYNCHONELLA. 1. R H Y N C H O N E L L A PSITTACEA (ROSTRUM-PSITTACI), Chemn. Anomia rostrum-psittaci, Ch., viii. p. 106, tab. 78. f. 4 a, b, c. B. psittacea, B. C. ii. pp. 22, 23 ; v. p. 164, pl. xcix. f. 4. Arctic and northern seas in both hemispheres, from Spitzbergen to Drontheim, and from Franklin-Pierce Bay in 79° 25 N. lat. to Maine, U. S., in the North Atlantic; and from Behring Strait to Japan in the North Pacific : 10-690 f. Australia, with R. nigricans (J. W . Flower)! Shetland, 90-100 f. ; apparently recent, but possibly relics of the " glacial era" (J. G. J). North Sea, S.E. of Yarmouth (Metzger) ; possibly also semifossil. Several spurious or more than doubtful habitats on the Scotch and English coasts have been recorded. 2. RHYNCHONELLA SICULA1, Seguenza, MS. (Plate XXIII. figs. 5,6.) B O D Y whitish, gelatinous : mantle not furnished at its edge with setae or bristles, as is the case in the Terebratulidae : arms fringed with short cirri, which are unequal in length and curl inwards at their extremities : byssal plug small, cylindrical, and slender; its outer case or sheath is chitinous, and resists the action of liquor potassae. S H E L L ovately triangular, having the upper valve convex and the lower valve somewhat compressed, rather thin, semitransparent, and glossy: sculpture, numerous and close-set, minute, very fine and regular longitudinal striae ; lines of growth slight but distinct: colour pale brownish-yellow: margins nearly straight in front (instead of being flexuous as in R. psittacea), irregularly sloping at the sides, broadly angular behind : beak very short, incurved : foramen very small, oval, incomplete: deltidium broad, slightly concave : hinge-plate narrow : teeth in the upper valve triangular and long; the processes in the lower valve approximate at the base, forming an inverted V, and are irregularly oblong, rather thin, and concave in the middle; each of these processes extends outwards as a thin and narrow lamina, which curves upwards, and is divided at its outer end in a pectinate or comb-like manner, as in Atretia gnomon, but has more digitations or points ; the sockets of the teeth are grooved transversely : septum ridge-like, short, placed in the centre of the 1 Sicilian. |