1878. J 'lightning' AND 'porcupine' EXPEDITIONS. 413 



the internal apparatus is different, and the beak is prominent in 

 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 Mannta 

 of Dewalque (Soc. Malac. Belg., 1874), from the "sable noir," or 

 Upper Miocene of the Antwerp Crag, is also a Rhynchonellidan ; but 

 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. 

 I. Rhynchonella psittacea (rostrum-psittaci), Chemn. 



Anomia rostrum-psittaci, Ch., viii. p. 106, tab. 78. f. 4 a, b, c. 



R. psittacea, B. C. ii. pp. 22, 23 ; v. p. 164, pi. 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 ou the Scotch and English coasts have been re- 

 corded. 



t/ 2. Rhynchonella sicula 1 , Seguenza, MS. (Plate XXIII. ties. 

 5, 6.) 



Body 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 potassse. 



Shell 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 B. 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 pro- 

 cesses in the lower valve approximate at the base, forming an in- 

 verted 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. 



