144 ARCID^. 



numerous fine striae which radiate from the beak, and a few 

 irregular lines of growth which form dark zones but do not 

 intersect the longitudinal striae ; these striae are wanting 

 on the lunule and dorsal area : colour greyish- white under 

 the epidermis, which is yellowish -green and very closely and 

 microscopically wrinkled in a transverse direction : margins 

 angular and somewhat truncate on the anterior side, curved 

 in front, and obtusely wedge-shaped and rounded on the 

 posterior side ; beaks prominent bat blunt, slightly recurved : 

 lunule marked obliquely by the lines of growth, and strongly 

 indented or grooved across below the beaks ; it is defined by 

 a slight furrow, and projects a little outwards, so as to form a 

 blunt and indistinct keel : ligament slight : cartilage oval, con- 

 tained in a short projecting cavity underneath the beaks : 

 hinge-line somewhat curved : Jiinge-plate as in the last species : 

 teeth sharply pointed and slightly recurved, about 15 on the 

 anterior, and 25 on the posterior side : inside nacreous and 

 highly iridescent, striated and notched as in N. sulcata, but in 

 the present species the crenulations extend to the posterior 

 angle : muscular scars oval and distinct. L. 0*475. B. 0*475. 



Yar. 1. radiata. Shell larger, flatter, more decidedly tri- 

 angular and produced at the posterior side, and marked with 

 numerous purplish-brown streaks, which radiate from the 

 beaks outwards. L. 0*6. B. 0*6. N. radiata, F. & H. ii. 

 p. 220, pi. xlvii. f. 4, 5, and xlviii. f. 7. 



Yar. 2. lumidida. Shell smaller, more triangular and con- 

 vex. N. tumidula, Malm, Proc. Scand. Soc. (1862), p. 621. 



Habitat : Common in sand and gravel on all our 

 coasts from Shetland to the Channel Isles, at various 

 depths ranging from 3 to 85 fathoms. Capt. Beechey 

 dredged it off the Mull of Galloway in 145 fathoms. 

 Var. 1. More local, but widely distributed in the British 

 seas, as well as from the Swedish coast (Malm) to 

 Algeria and Sicily (M f Andrew). Var. 2. Off Unst, in 

 85 fathoms ; very rare. This variety is also Scandi- 

 navian, and was obtained by Dr. Torell in 80 fathoms. 

 The typical form and the first variety occur in all the 

 upper tertiaries, both in Great Britain and the south of 

 Europe. Beyond our shores the recent species inhabits 



