LAMELLIBRANCHIATA. ^^ 



size, only in a few does the inaioin of the left valve extend a trifle beyond that of the right, without 

 however bending over it" and p. 453 (under J'<c/</i Jhskyiisi Forbes var. major L,eche): "in all very 

 thin-shelled /Wfrn-iorms the weakest valve gives way at the edge, when the animal retracts strongly 

 on dying, thus producing the characteristic concavity, which runs concentrically with the margin of the 

 shell, the sculpture markings on the right valve giving this a greater firmness". 



In his diary written on the Danish East Greenland Expedition of 1900, the young zoologist 

 Soren Jensen, since dead, entered the following observations regarding P. groenlandicus : ''.... this 

 small bivalve is able to swim when full_\-dcveloped. It opens and sliuts the valves, beating the 

 water out during the latter process with considerable force and thus moving backwards through 

 the water. The specimens which lay on the bottom of a glass with water, could in this way "gape" 

 their way right up to the surface". 



Pecten frigidus Jensen. 

 PI. I, figs. 7 a— f. 



1S76. Pedcn fragiiis Jeffreys, Ann. Mag. Nat. Hist. (4) XVIII, p. 424 (partim). 

 1877. P.fragilis Friele, Nyt Mag. f. Naturvidensk., 23 Bd., 1877, II, p. 2. 

 1879. P.fragilis Jeffreys, Proc. Zool. vSoc. Lond., p. 561 (partim), PI. 45, fig. i ad dextram. ') 

 1879. P.fragilis Friele, Catal. d. anf d. norw. Nordmeerexped. bei Spitzbergen gef. Mollusken; Jahrb. 

 Deutsch. ;Mal. Gesellsch. VI, p. 264. 



1901. P.fragilis Friele & Grieg, The Norwegian North Atlantic Exped., Zool., MoUusca III, 



1 901, p. 8. 



1902. P. biscayensis Friele (non Locard), Moll. d. ersten Nordmeerfahrt d. Fischereid. "Michael Sars" 



1900; Bergens Museums Aarbog 1902, No. 3, pp.3, 15 & 17. 

 1904. P. frigidus Jensen, Vidensk. Medd. fra den naturhist. Foren. i Kbhvn. 1904, p. 305 (cum fig.). 



1904. P.fragilis Hagg, Arkiv for Zoologi, Bd. 2, 1904, No. 2, p. 30. 



1905. P. frigidus Bavay, Mem. de la Soc. Zool. de France, T. XVII, 1905, p. 189, PI. 17, fig. 4. 



The shell a little higher than long, irregularly suborbicular, with the anterior and lower 

 margins forming together a semicircle and the posterior margin sligthly arched or almost straight, 

 compressed, the right valve flatter than the left, translucent silvery white. The valves very tliiu, 

 fragile, with concentric folds, to the number of about 12 in the adult, broad in the middle of the 

 shell, narrowing towards the lateral margins, with numerous fine, elevated, radiating strise. The 

 auricles small, unequal, the posterior the smallest, faintly marked off from the shell, the anterior 

 distinctly marked off from the shell, the left triangular, the right with an acutely angulated sinus for 

 the byssus. Hinge-margin straight, pit for the cartilage very small, triangular; the internal surface 

 shining. Length 27"""., height 29"""., breadth 6.5""". 



Distribution: The ice-cold depths of the Norwegian vSea, from Spitzbergen down towards 

 Iceland, the Fseroes and Shetland, 579 — 1539 fm. 



') The figure to the left represents, so far as the contour is concerned, the same valve seen from the inside, but 

 provided with a form and sculpture as if seen from the outside. 



The Ingolf-Expedition. II. j. 5 . 



