56 



LAMELLIBRANCHIATA. 



Elsewhere D. vitrcum ranges from the Kara Sea and Spitzbergen to the Mediterranean and 

 the Azores; on the American side from the Gnlf of St. Lawrence to Campeche. It is said to occur 

 down to a depth of 2750 fm., and in arctic regions it reaches as high np as 10 fm. (cf. under East 

 Greenland). 



Idas argenteus Jeffreys. 

 PI. Ill, figs. 3a — e. 

 Idas argenteus Jeffreys, Ann. Mag. Nat. Hist. (IV), vol. XVIII, 1876, p. 428; Proc. Zool. Soc. 1879, 

 p. 570, PI. 45, fig. 3; Proc. Zool. Soc, 1882, p. 683. 



Shell having the shape of an irregular parallelogram (varying from rhomboidal to oblong), of 

 a delicate texture, rather opaque, iridescent; it is covered with a pale brownish-yellow periostracum, 

 which rises into fibrous excrescences on the posterior side; under the periostracum the shell is silvery 

 white, except the beaks which are reddish brown ; sculpture, very fine and close-set transverse striae 

 and microscopic longitudinal striae, radiating from the beaks; margins straight at the back and in 

 front, rounded on the anterior and smaller side, and sloping from the back with a curved outline on 

 the posterior side; beaks circular and incurved, placed near the anterior side; an internal and long 

 cartilage covers the hinge; hinge-line nearly straight, but obtuse-angled at the hinge; hinge-plate 

 narrow, minutely and closely denticulated on both sides of the hinge; inside polished and nacreous; 

 edge plain; scars inconspicuous. Size of the largest specimens about 8""". 



In 1876 Jeffreys described the species almost as above, the additions and changes made by 

 himself in 1882, however, being added. 



This small M\tilid was taken by the "Ingolf at: 



St. 67. S. of Iceland 975 fm. 3° C. ca. 260 spec. 



The specimens of the "Ingolf'-Expedition agree extremely well with the descriptions of Jeffreys. 

 A radiating striation however can hardly be seen and the hairy periostracum is often distributed over 

 a greater part of the shell, mostly however on the posterior and dorsal surfaces. 



Distribution. During the "Valorous" Cruise of 1S75 one valve was taken in the North 

 Atlantic (56°ii'N. L., 37°4i' W.L.I at 1450 fm. The "Porcupine" Expedition of 1869 also found a shell 

 in the Bay of Biscay at a depth of 994 fm. Lastly, it was taken on the cruise of the "Triton" between 

 the Hebrides and the Fseroes at 516 fm., "inhabiting deserted tubes of Teredo megotara in a large 

 water-logged piece of pine-wood, to which the Idas had fixed itself by a strong byssus". The specimens 

 of the "Ingolf'-Exped. were taken south of Iceland (6i°3o' N. L., 22°3o' W. L.) at 975 fm., under similar 

 conditions to the last; the trawl brought up two large pieces of pine-wood, which had been pierced 

 through and through by Teredo; in some of the Teredo tunnels were in addition mud and worm- 

 tubes, further worms and small bivalves, namely Idas argenteus. 



In 1882 Verrill mentions an "Idas argenteus Jeffreys van? lamellosus Verrill (perhaps sp. 

 nov.)", taken by the "Fish Hawk" in 1881 at New England off Martha's Vineyard (S. S. W. V4W., 

 i03'/j miles) at a depth of 335 fm.'). On this Jeffreys (I.e. 1882) makes the following comment: 



>) E..\. Verrill: Catal. of Marine MoUusca added to the Fauna of New England; Trans. Conn. Acad. vol. V, 1882 (p. 579). 



