36 NUCUMD.*. 



dato-subtruncata. Margo dovsalis anticus arcuatus, declivis; 

 posticus rectiusculus, vix declivis. Margo veutralis subarcuatus, 

 iitriiique subretusus. Arese dorsales liaud circumscripta!. — Oval, 

 convex, yet compressed behind, much gaping at the front ventral 

 margin, and less so posteriorly, with an almost imperceptible um- 

 boual ridge, clothed by a highly polished olive epidermis, whose 

 zones of increase are more intense or blackish, smooth, except for 

 the wrinkles of growth. xVnterior side slightly the longer, almost 

 symmetrically rounded : posterior side bluntly peaked, rounded- 

 subtruncate at the extremity, which is subcentral, but projecting 

 more above the middle. Front dorsal edge arcuated, and mode- 

 rately sloping : hinder dorsal edge straightish, and not much 

 sloping. Ventral margin subarcuated, but with a slight anterior 

 retusion, and a still more trifling posterior one. Dorsal areas 

 undefined; the hinder dorsal lips pouting. 



61. L. Myalis, Coidlmuy, f. IS. — T. speciei prsecedenti assi- 

 milis, sed latere antico longiore minusque rotundato, extremitate 

 lateris postici minus rostrati magis truncata, et area magna et 

 acute circumscripta. — Resembling the last, but with a well marked 

 large escutcheon, tliat is sharply defined by its abrupt retusion. 

 The lunule, too, though indistinct, is wrinkled by sulci. There 

 is no vestige of an umbonal ridge, and the posterior compression 

 is less marked. The anterior side is decidedly the longer, and 

 blunter (less rounded) at its extremity : the hinder termination is 

 less peaked and more truncated, the extreme projection is, for the 

 most part, above the middle. The epidermis, which ranges from 

 yellowish-olive in the young to dusky-olive in the adult, is less 

 polished. The front dorsal edge is straighter : the ventral margin 

 rather less arched, and the refusions are still less apparent. The 

 cartilage-pit is broadly triangular ; the teeth numerous on cither 

 side.* 



62. L. WoODWARDi, Ilanleij, i. 17, 22. — ^T. pertenuis, valde 

 compressa, valde inajquilateralis, subelliptica, postice brevis et 



* I am unable to procure a specimen of the A^. Gouliii of Dekay (New York 

 Moll. p. 180, f. 221), which is said to resemble this species in general form! but to 

 diflcr in the nmnber of its teeth and in the posterior side not being angular. I con- 

 dense its description, which was derived from one individual now in the State col- 

 lection. — 



Thin, ovate, subequilateral, with an olive-green epidermis, which has a few paler 

 conecutric lines, that become still lighter behind ; with faint concentric stria; : I'roEit 

 dorsal margin slightly curved : posterior margin slightly beaked, with three or four 

 imbricated stria; on the sides, extending from the beaks to the margin, where the im- 

 brications are most ap))areut. 4— |. Long Island Souud, New York. Eigiiteen 

 teeth were present in each vahe. 



