NucuLiD^:. 57 



(certainly not strictly ovate, and by no means inequilateral for its 

 genus,) the reader is referred to our engraving of that example, 

 which closely resembles tenuis aud injlata, but appears distinct. 



32. N. M'Akdr,«i, Eanleij, f. 143.— T. anguste subovata, in 

 medio latior, pertenuis, semipellucida, valde compressa (nisi ad 

 umbones), cute nitidissima cinerea induta, undif[ue loevis. Ex- 

 tremitas antica superne rotundato-obtusangularis ; postica minime 

 brevis (pro genere ejus), fere in medio rotundato-acutangularis. 

 Margo dorsabs antice rectiusculus haudque declivis ; ventralis 

 integer, multum arcuatus, postice multum acclivis. Nates per- 

 acutfE, hand tamen valde eminentes. Arese dorsales subimpressje, 

 baud circumscripta; lunulsc labiis carinato-protrusis. — Produced 

 subovate, very thin, much compressed, except on the umboes, with 

 a polished cinereous epidermis, everywhere smooth. Anterior side 

 rotuudately obtusangular above, the dorsal edge being horizontal 

 aud straightish, well rounded below ; posterior side long for the 

 genus, rotundately acutangular, the peak subcentral, the dorsal 

 slope straightish, not abrupt : chief breadth subcentral. Veutral 

 margin entire, much arcuated, much rising behind. Beaks not 

 much projecting, yet conspicuous from their acuteness. Dorsal 

 areas undefined, yet somewhat concavely impressed. Lunular 

 lips carinately pouting. — The hinge resembles that of tenuis, but 

 the teeth seem scarcely so numerous. I doubt, however, the ma- 

 turity of the examples. 



33. N. TENUIS,* Mont., f. 140, 141. — T. pertenuis, undique 

 (etiam ad umbones) compressa, antice semiovalis, postice brevis- 

 sima et rotundato-biangulata ; cute nitidissima olivaceo-lutescente 

 vel olivaceo-cinerea induta, lineis inerementi tantum insculpta. 

 Margo dorsalis anticus prope nates acutas, perparvas, valde obU- 

 quas,vixprominentes, rectiusculus vixque declivis, deinde arcuatus 

 et declivis ; ventralis integer, ante medium valde arcuatus. Areae 

 dorsales miuime definita;; lunulse labiis protrusis. Dentes cir- 

 citer 15-7. — Very thin, compressed (even at the umboes), about 

 half as long again as broad, chiefly expanded anteriorly at about 

 one-third the entire length, semioval in front, rotundately biangu- 

 lated behind, with a polished epidermis, that ranges in tint from 



* The iV. tenuis oi Gould (Invert. Massai-h. p. 105, f. 64.— Dekay, N. York Moll. 

 181), described from scarcely matured examples, is exactly intermediate in characters 

 between the typical tenuis and the boreal iiijhita. It is more produced lengthways, 

 less swollen, and usually greener than the latter -, more trapezoidal, less arcuated 

 ventrally, and less rising aud projecting at the lower posterior corner than the former. 

 Gould states that the manuscript name lucida was attached to the specimens in the 

 Philadelphia Museum, and it is doubtful whether it should be rcjardcd as a distinct 

 species, or as a mere varietal link with iii/tnln. 



