FAMILY MYTILIDiE MYTILUS. 181 



N. tenuis. (Gould, 1. c. p. 105, pi. 54.) Shell small, thin, trapezoidal, smooth, without radiating 

 lines: beaks prominent, placed anteriorly; margin simple. Teeth long and slender, about eight 

 behind and four or five before the beaks. Color : epidermis grass-green. Length, 0-25 ; transverse 

 diameter, O-S. Stomachs of fishes. 



N. minuta. (Gould, 1. c. p. 101. N. ienuisulcata, Coutiiouy, Bost. Jour. Vol. 2, p. 64, pi. 3, 

 fig. 8. Pl. 12, fig. 213 of this book.) Shell ovate, lanceolate, inequilateral, posteriorly much 

 narrowed and rostrated : surface with numerous concentric ridges. Teeth twelve before and sixteen 

 behind the beaks. Color: epidermis light greenish yellow. Length, 0-9; transverse diametef, 

 l-O. Stomachs of fishes. 



N. acuta. (Conrad, Mar. Conchol. pl. 6, fig. 2.) Shell very small, ovate-elongate, convex, with 

 numerous concentric stria;. Beaks behind the centre pit, very small. Width, 0'2. This was 

 found in so very recent a fossil deposit, as to induce Mr. Conrad to suppose that it may still be 

 found on the coast, but overlooked on account of its size. Virginia. 



FAMILY MYTILID^. 



Animal oval, moderately thick, with its mantle open throughout its lower portion and 

 adhering towards its edges ; a separate opening behind for the excrements, forming very 

 rarely a tube. Foot tongue-shaped, channelled, and with a byssiis behind. With a very 

 few exceptions, marine. Shell usually with an epidermis, equivalve, very inequilateral. 

 Hinge without teeth; ligament linear, marginal, partly included: jjosterior muscular 

 impression very small ; the anterior large. 



GENUS MYTILUS. Linnccus. 



Animal with the lobes of the mantle fringed about the opening of the vent. Mouth moderately 

 large, with two pair of soft triangular labial appendages. Foot slender, cylindrical, with 

 a silky byssus at its base and posteriorly. Shell longitudinal, subtriangular ; apex acute, 

 pointed at base, and fixed by a byssus. Beaks terminal, pointed, nearly straight. Hinge 

 lateral, usually without teeth ; ligament marginal, deeply seated, rectilinear, partly internal. 

 Muscular impressions elongated, club-shaped ; the anterior largest : palleal impression entire. 



Obs. The species of this and the following genus are popularly known under the name of 

 Mussels. 



