90 MOLLUSCA FROM THE CRAG. 
2. Lepa MYALIS, Couthouy. Tab. X, fig. 17, a—e. 
Nucura myauis. Couth. Bost. Journ. Nat. Hist., vol. ii, p. 62, pl. 3, fig. 7, 1839. 
— — Gould. Invert. of Massach., p. 99, 1841. 
— —_ Dekay. Hist. New York Zool., p. 180, pl. 13, fig. 219, 1843. 
—  ostonca? Woodward. Geol. of Norf., p. 44, 1833. 
— allied to optonea. Lyell. Mag. Nat. Hist., New Series, p. 328, 1839. 
—  optoncoiprs. S. Wood. Mag. Nat. Hist., New Series, vol. iv, p. 297, pl. 14, 
fig. 4, 1840. 
— — Morris. Cat. Brit. Foss., p. 95, 1843. 
—_ — Forbes. Mem. Geol. Surv., vol. i, p. 420, 1846. 
— HYPERBOREA. Lovén, sec. Gould. 
Youpta aNGuLARIS. Moller. Ind. Moll. Groenl., p. 19, 1842. 
Spec. Char. Testa transversd, inequilaterd, elongato-ovatd, tenu; anticé elliptico- 
rotundatd, postice subrostrata ; levigata ; dentibus mediocriter angulats. 
Shell transverse, inequilateral, elongato-ovate, thin, and externally smooth ; 
anterior side roundly elliptical, posterior subrostrated; externally smooth; teeth 
moderately angulated. 
Longitudinal diameter, \# inch. Height, | inch. 
Locality. Red Crag, Sutton and Butley. 
Mam. Crag, Chillesford and Bramerton. Recent, North America. 
This species, like the preceding one, appears in the recent state to be confined 
to the colder regions of the globe, and as a fossil has been found only in the 
newer Tertiaries in this country. In the Red Crag I have met with but very 
few specimens, while at Chillesford it may be obtained in abundance, and is by no 
means scarce, I believe, in the Estuary portion of the Mammaliferous Crag at 
Bramerton. It is readily distinguished from JZ. /anceolata, in being more equi- 
lateral, the posterior portion is comparatively larger, and it wants the peculiar 
diagonal ridges which ornament the surface of that shell; nor does it appear 
even to attain to such dimensions, my largest specimen being under two inches in 
its transverse or longest diameter, neither is it ever so thick a shell. A specimen 
obtained in the Red Crag at Sutton is more elongated or attenuated than those 
usually found at Chillesford, and it approaches in that character Z. Himatula of the 
American Seas (fig. 17, c); but that species is rather more elegantly formed, and 
more rostrated, with a greater curvature at the posterior termination. Our shell is 
covered with concentric strie or lines of growth, but not in regular ridges, and there 
is not so distinct a sinus on the anterior side, as in LZ. /anceolata, although in some 
specimens a radiating line traverses that side of the shell from the umbo, which line is 
produced by a slight interruption to the regular curve of the margin: a large corselet 
is well defined on the posterior side, and the ligamental pit is comparatively larger 
than in ZL. limatula. The shell is sometimes thickened in the interior, though never 
