IGO ARCADE. 



been kindly furnished me by Colonel Totten, which he took by 

 dredging in Provincetown Harbor. Eastport, Grand Manan (^Stimp- 

 sori) ; Fishing Banks ( Willis') ; Labrador {Packard} ; Long Island 

 Sound (<S'. Smith) ; Northumberland Sound {Belcher). 



This is the living analogue of the fossil species described by Mr. 

 Say under the name of N. Iccvis, in " American Concliology," pi. 12. 

 In size, shape, and the position of the beaks they accurately agree ; 

 but the fossil species wants the emargination or flexure under the 

 posterior tip. Y. myalis is greater in height, thicker, darker, and 

 has the beaks about as far removed towards the posterior as they 

 are towards the anterior extremity in this shell. 



Eroded spots, filled with a black substance, are frequently seen 

 externally, which are marked by a corresponding chalky opacity 

 within. 



In its shape, and the perfect polish of its surface, this shell resem- 

 bles the seed of the Sapotilla {Achras sapnta), a tropical fruit; and 

 I have substituted that name instead of N. IcEvig-ata, under which I 

 gave the specific characters of the shell in the " American Journal 

 of Science," as I find that name preoccupied. 



[Though the specific value of the shell here described has been 

 thus far unchallenged, ujjon an examination of many specimens I 

 am not fully satisfied that it is anything more than an abbreviated 

 variety of Y. limatula. 



Yoldia myalis. 



Shell ovate, smooth, olive colored ; anterior part longest and rounded ; poste- 

 riorly acuminated, and sub-rostrated ; teeth about twelve on each side. 



Nuculn wijalis, Cotithouv, Bost. Journ. Nat. Hist. ii. 61, pi. 3, fig. 7 (1838). — Goui.D, 

 Inv. Mass. 1st ed. 99. — De Kay, Nat. Hist. New York, 180, pi. 12, fig. 219. — Sow- 

 KRBY, Thes. Conch. (Nncii/a) pi. 1, fig. 18. — Mighels, Shells of Maine, 17, and 

 Journ. Bost. Sec. iv. 



Lfda mi/nlis, Stimpsox, Shells of New England, 10 (18.51) ; Inv. Gr. Manan, 20. 



Nitciilu lijjperborea, Loven. 



Shell ovate, thin, slightly gaping at both extremities, moderately 

 convex ; surface somewhat undulated by distant 

 concentric ridges, and covered with an olive- 

 colored epidermis, arranged in alternate darker 

 and lighter zones ; beaks not elevated, a little 

 behind the middle ; anterior part semi-elliptical ; 

 posterior part sub-triangular, upper margin be- 

 hind the beaks straight, compressed and sharp to the very tip, which 



