176 MOLLUSCA FROM THE CRAG. 
ASTARTE BOREALIS. Forbes. Mem. Geol. Sury., vol. i, p. 413, 1846. 
— compressa. Macgill. Moll. Aberd., p. 261, 1843. 
— — Thorpe. Brit. Mar. Conch., p. 247, 1844. 
—  cyprinompEs. Duval. Révue Zool. Cuy. Soc., p. 278, 1841. 
== — Hanley. Recent Shells, Suppl., pl. xiv, fig. 40. 
— corruGata. Loven. Ind. Moll. Scand., p. 37, 1846. 
— — Middendorff. Mém. de YAcad. Imp. St. Petersb., p. 562, t. xvii, 
figs. 4—10, 1849. 
— arctica. Moller. Ind. Moll. Groenl., p. 19, 1842. 
— — Forb. and Hanl. Hist. Brit. Moll., vol. i, p. 461, pl. xxx, fig. 7, 1848. 
— actna. Brod. and Sow. Zool. Journ., vol. iv, p. 365, 1829. 
— — Gray. App. to Beechey’s Voy. Zool., p. 152, t. xliv, fig. 19, 1839. 
—  sEMISULCATA. Gray. List Brit. Moll., p. 92, 1851. 
Crassina ArcTICA. Gray. Append. to Parry’s Voyage. 
— — Hanley. Recent Shells, vol. i, p. 88. 
— BoREALIS. Nilson. Nov. Act. Holm., p. 188, pl. ii, figs. 3, 4, 1822. 
= compressa. Brown. lust. Brit. Conch., pl. xviii, figs. 4, 5, 1827. 
— corruGATA. Id. a a pl. xvi, fig. 4 AS 
— Wirnamr. Smith and Brown. Mem. Wern. Soc., vol. viii, p. 57, pl. i, figs. 24, 25, 
1838. 
— SEMISULCATA. Leach, in Ross’s Voy., Append., p. 175, 1819, (fide Gray.) 
TRIDONTA BOREALIS. Schum. Essai d’un Nouv. Syst. des Hab. des Vers. Test., p. 146, 
pl. xvii, fig. 1 a—, 1817. 
Spec Char. Testa crassd, cordato-trigond, inequilaterali, postice longiore, subcompressd, 
lunuld elongata, profundé excavatda ; margine integerrimo. 
Shell thick, somewhat triangularly heartshaped, slightly compressed, inequilateral, 
posterior side the longer ; a deep and elongated lunule ; margin smooth. 
Length, \} inch; height, 1} inch. 
Localities. Mam. Crag, Bramerton, Bridlington. 
Clyde Beds, Opslo, Uddevalla. 
Recent Arctic Seas, Britain, Nova Zembla, Behring Straits, (4/ddendorf.) 
This appears to be comparatively a modern species. In its living state it is deci- 
dedly a Boreal shell, although it is occasionally obtained alive in our own seas. As a 
fossil it is not at all rare in this country, either from Norwich or from the Clyde beds. 
Like most of the species of this genus, it is subject to considerable variation, not only 
in its outward form but in the markings of its exterior. 
The specimens in my Cabinet from near Norwich, are generally uniform in shape, 
and when not decomposed, the outside is nearly smooth, with scarcely the appearance 
of ridges, but in the Bridlington shells, a very material difference is exhibited, even in 
the few specimens that I possess. The more common form is somewhat rounded, 
having a height only a little less than the length, but that which has been called 
A, Withami is not only comparatively much longer, but has the posterior side sloped off 
to a rounded point (fig. 2c, 7), and this has the umbo covered with distinct ridges 
extending over a fourth of the exterior, while the one from Norwich is in that part 
