56 



LAMRLLIBRAXCIIIATA. 



Elsewhere D. vitreum ranges from the Kara Sea and Spitsbergen to the Mediterranean and 

 the Azores; on the American side from the Gulf 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 up as 10 fm. (cf. under East 

 Greenland). 



Idas argenteus Jeffreys. 

 PI. Ill, figs. 3 a — e. 

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

 p. 570, PL 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 mm . 



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

 himself in 1882, however, being added. 



This small Mytilid 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 1875 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 Faeroes 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'\V. 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 18S2 Verrill mentions an -'Idas argenteus Jeffreys var.? lamellosus Verrill (perhaps sp. 

 nov.)", taken by the "Fish Hawk" in 1881 at New England off Martha's Vineyard (S. S. W. r / 4 W., 

 103' ., miles) at a depth of 335 fm. 1 ). On this Jeffreys (I.e. 1882) makes the following comment: 



1 ) K A. Verrill: Catal. of Marine Mollusca added to the Fauna of New England ; Trans. Conn Acad. vol. V, 1882 (p. 579). 



