140 ARCID.E. 



Genus I. NU'CULA*, Lamarck. PL IV. f. 1. 



BODY roundish-oval, somewhat compressed : mantle open in 

 front and at both sides : gills unequal in size, one pair over- 

 lapping the other : lips or palpi long and pendulous : foot oval 

 and having its margin serrated or notched. 



SHELL triangular and compressed, highly nacreous : margin 

 of the posterior side rounded : lunule or area below the beaks 

 heart-shaped : ligament partly internal : cartilage internal and 

 contained in a spoon-shaped cavity : teeth sharp and recurved, 

 those on the anterior side being fewer than those on the other 

 side : pallial scar entire. 



The Nuculae inhabit mud, sand, and gravel in all the 

 marine zones on our coast ; and they appear to be gre- 

 garious. They are found in every degree of longitude 

 and latitude throughout the globe. The umbonal area, 

 or that part of the shell which is terminated by the 

 beak, is often eroded and the nacreous layers exposed, 

 probably owing to the action of sulphuretted hydrogen, 

 which is evolved from animal matter in a state of de- 

 composition contained in the mud. Sometimes the 

 beak is encrusted with a ferruginous or mineral deposit. 

 The lunule in the present genus and Leda projects con- 

 siderably, so as to resemble a pair of pouting lips. Each 

 organ (in the mollusk as well as in Woman) encloses a 

 row of dazzling white teeth. But here the analogy ends. 

 Such comparisons of natural beauty constitute one of 

 the minor charms of science, and may be pardoned in 

 an enthusiastic naturalist. 



I am not prepared to accept D'Orbigny's proposition, 

 which has been adopted by Gray and other concho- 

 logists, that this genus ought to form a distinct family. 

 The genus Nuculana of Searles Wood resembles Nucula 

 in shape, but has teeth like those of Pectunculus, and no 

 * A small nut. 



