164 MYAD^E. 



in a perfectly fresh state by Mr. Howse, of Newcastle-on- 

 Tyne, who kept it alive several days; it was taken off the 

 Fern Islands, on the coast of Northumberland,, in deep water, 

 in March 1852. It is closely allied to My a, from which it 

 differs in having a narrower foot, with a byssal groove ; there 

 is also a more developed tooth, and the ligament is much 

 more external. But it is still closer to the genus Saxicava, 

 of the Gastrochanida, scarcely differing therefrom ; the hinge 

 of which is all but identical, having the small conical tooth in 

 each valve ; the adductor, pallial and siphonal cicatrixes nearly 

 agree in shape, colour, and position ; the character of the foot 

 is similar, and many of the Saxicavce are often found with as 

 proportionately large posterior and anterior ventral gapes : the 

 principal difference is in the aspect of the tubes of the two, 

 which in this species is the counterpart of that of My a truncata, 

 whilst in Gastrochaena and Saxicava they appear as two tubes 

 soldered together, showing sometimes the real divisional line 

 marking the extent of the internal wall between them, and 

 sometimes only an apparent depression on the common sheath. 

 This comparison of the two genera would appear to show that 

 the appellation of Panopaa to our unique British species is 

 almost superfluous, and that it might have merged in the 

 Myadae or Gastrochamda. 



MYAD.E. 



This family only contains the genus Mya, with a very few 

 species, Lutraria being merged in it ; it is allied to the Pho- 

 lades by the characters of the branchiae, the closure of the 

 mantle, and the powerful siphonal apparatus ; the generalities 

 of the anatomy scarcely differ. The so-called Lutrariae have 

 been associated with the Mactrida, but they have very little 

 similarity, except in the shape of the foot, and the triangular 

 or V-shaped tooth ; the mantle of the one is entirely closed 

 ventrally, and has exceeding long siphons ; in the other it is 

 open, as in the Veneres, with very short tubes. 



