158 AKCAD^. 



upper margin of the other triangle is nearly direct, a little upturned 

 or beaked, and its edge is compressed into a sharp crest, the pos- 

 terior margin moderately rounded, joining above and below with an 

 abrupt curve of a truncated appearance ; the anterior portion is 

 inflated, and the epidermis of a dusky green ; the posterior portion 

 is compressed, has one or two faint waves or radiations, and the 

 epidermis is of a lighter yellowish-green, minutely wrinkled ; surface 

 coarsely marked with lines of growth, and covered with a sooty 

 powder, which easily rubs off and leaves a glossy black. Hinge 

 with a very large, spoon-shaped cavity, and on each side of it are 

 about twelve teeth, each one folded from the centre to an angle of 

 45° ; cavity of the beaks very capacious ; interior polished white, 

 impressions faint, the pallial one with a deep sinus. Length, two 

 and four fifths inches ; height, one and thirteen twentieths inches ; 

 breadth, one and one tenth inches. 



Animal with the mantle thin edged, a large foot divided by a deep 

 groove, the edges when expanded simple ; the lal)ial organs very 

 large and protruding from the ventral margin. 



First taken by Dr. Storer from the stomachs of Pkuroncctcs 

 dentata, or sand-dab, caught off Race Point, in December, 1837. In 

 that winter a dozen or more specimens were taken from the same 

 species of fish, and from the same locality ; but last winter they 

 were sought for without success. Fishery Banks ( Willis) ; Duck 

 Island, Grand Manan (^Btimpsoii) ', Greenland (^Mailer). 



It is a very extraordinary shell, far exceeding in size any known 

 species of the genus. Indeed, its peculiar shape, very large spoon- 

 hinge, and the teeth folded like those of Area, entitle it to a generic 

 distinction so far as the shell is concerned ; and I believe that a 

 knowledge of the animal will establish its claims to one. 



The young form was thus described as Nucula navicularis in the 

 first edition : — 



Shell small, thin, fragile, of a crescentic or somewhat kidney- 

 shaped form, smooth, tumid at the beaks, and compressed at the 

 sides, slightly gaping at both ends ; beaks very nearly central, 

 prominent, and directed backwards ; anterior side elliptically round- 

 ed ; posterior side somewhat narrowed and compressed, and very 

 slightly truncated ; basal margin strongly curved ; surface smooth, 

 with very indistinct lines of growth ; epidermis a light pea-green, 

 thin, with many eroded spots. Ligamentary fosset broad, prom- 

 inent, and oblique ; teeth about eight before and ten liehind the 

 fosset. Interior glossy white ; margin simple. Length, eleven 



