DIPLODONTA. 253 



think that the coating arises from a deposit of faecal 

 matter in still and deep water, and not from any pecu- 

 liar secretion of the animal, or from a mineral or che- 

 mical deposit. 



It is the Cryptodon rotundatum of Searles Wood's 

 1 Catalogue of the Crag Mollusca/ Judging from an 

 examination of a single valve so named by Forbes, his 

 Kellia abyssicola from the iEgean appears to be only 

 the decorticated state of the present species. 



Genus IV. DIPLODON'TA * Brown. PI. V. f. 7. 



Body nearly globular, but compressed : mantle thick, having 

 a plain margin : tuhes none : gills two on each side : labial 

 palps the same : foot lancet-shaped. 



Shell squarish, convex, and smooth: beaks not much re- 

 curved : lunule inconspicuous : ligament quite external : teeth, 

 two cardinals in each valve, the anterior of which in the right 

 valve is simple, and the other bifid or cloven, the contrary being 

 the case in the left valve ; laterals laminar and indistinct. 



The peculiarity of this genus consists in there being 

 two separate gills and as many labial palps on each side, 

 instead of a single gill and palp as in all the other genera 

 of the same family, and also of the ligament being alto- 

 gether external, and the cardinal teeth more numerous 

 and symmetrical. In adult specimens the anterior 

 muscular impression is elongated, like that of Lucina. 



The Diplodontce inhabit every part of the marine 

 world ; and many fossil species have been described. 

 Torell has lately dredged at Spitzbergen a new species, 

 which is nearly allied to the only one we have in the 

 British seas. 



It is the genus Mysia of Leach, and (according to 

 Woodward) Sphcerella of Conrad. 



* Double-tooth. 



