MARSHALL: ON A BRITISH SPECIES OF MYRINA. 169 



is more incurved, with central rays or riblets, and the hinge-line is 

 crenated on one side only of the ligament. It also resembles Idas 

 argenteus, Jeffreys,- but this is very much larger, the lower margin is 

 concave, the epidermis does not rise into fibrous excrescences on the 

 posterior side, and the hinge-line is striated on one side only. Gwyn 

 Jeffrey's figure must not be taken as correctly representing /. argenteus, 

 that species having been described and figured from a decorticated 

 valve, which could not exhibit ,the fibrous epidermis, in addition to 

 which, the beaks are shown small and pointed, while they should be 

 incurved and invisible, with large obtuse umbones as in Modiola, and 

 similar to Myrina coppingeri and M. simpsoni ; while it must also be 

 remembered that I. argenteus is variable in its outlines, in consequence 

 of its peculiar adaptability to habitat. In specimens of the same size 

 the two are very closely allied, and in a strong light some examples 

 of 7. argenteus are found to have rays down the centre of each valve 

 corresponding to the strengthening riblets of M. simpsoni. 

 < Idas of Jeffreys is no more than a synonym of Mgrina, H. and A. 

 Adams. A valve was dredged in mid-Atlantic by the " Valorous " in 

 1450 fathoms, and another on the coast of Portugal by the "Porcupine" 

 in 994 fathoms (not Bay of Biscay, wrongly recorded by Jeffreys). 

 In describing the genus from these two valves, Gwyn Jeffreys ventured 

 to describe the ligament or cartilage as external, and in describing the 

 species (/. argenteus) he wrote "ligament not observable, the specimens 

 being imperfect single valves, but certainly not internal"; while Mr. 

 Edgar A. Smith, relying on this statement when describing his Mgrina 

 coppingeri, separates it, and rightly so, from Idas because though it 

 "has the hinge-plate similarly crenated, the ligament is described as 

 external," overlooking Gwyn Jeffrey's account of the discovery of 

 living specimens "between the Hebrides and Faroes in 516 fathoms, 

 inhabiting deserted tubes of Teredo megotara in a piece of pine-wood, 

 and in which the Idas were affixed by a byssus. * * * An internal 

 and long cartilage covers the hinge, and I was mistaken as to this 

 when I described the species from two small valves." So that the 

 genus Idas would appear to be quite superfluous. The author had 

 originally written " Perhaps allied to Myrina, although that genus has 

 an internal ligament and wants the hinge-plate crenated." We have 

 seen that the former attribute was an error, and the latter cannot be 

 considered a generic distinction seeing that it is a character equally 

 shared by Modiolaria, Dacrydium, and Crenella, in addition to which 



2 " Valorous ".MoUusca, Ann. Mag. N. Hist., 1876, p. 248; and Moll. " Lightning" and 

 "Porcupine," Proc. Zool. Soc, 1882, p. 683, pi. xlv, fig. 3. 



