88 MOLLUSCA FROM THE CRAG. 
within the margin of the shell, many of these have the posterior portion, as it were, 
cut off or wanting, in which the animal is entirely without the protruding siphonal 
tubes, and the impression of the mantle is therefore entire, this section constitutes our 
genus JVucula, the type of which is Arca nucleus, Linn. Many species formerly 
included have a prolonged posterior side, making the shell sometimes nearly equi- 
lateral, and the animal is then furnished with elongated siphonal tubes. This was 
originally proposed by Schumacher, under the name of Leda, without his being at all 
aware of the essential difference, as the only reason assigned for the division was that 
these shells were more nearly related to Pectunculus (Essai d'un Nouv. Syst. des Vers. 
Test. p. 173). 
Miller divided these latter or bilateral Nucule into two genera, without, however, 
any apparent distinction, either in the shell or animal; MVucula arctica, the species he 
intended as the type of his genus Yo/dia, being furnished with a sinuated impression 
like that of IV. minuta (the typical form of Zeda), indicating the possession of 
protruding siphons in the animal of that species: neither does the form of the exterior 
present any essential difference. 
These resemblances were more especially pointed out by Professor E. Forbes, in 
his valuable essay in the first vol. of the ‘ Memoirs of the Geological Survey,’ p. 418, 
where the two genera are united. 
Shells possessing the form and characters assigned to this genus are found in some 
of our oldest formations, and are continued through the more modern Periods. 
]. Lepa LAnceoxata, J. Sowerby. Tab. X, fig. 16, a—dé. 
Nucuna LANcEoLata. J. Sow. Min. Conch., t. 180, fig. 1, 1817. 
— —_ Morris. Cat. of Brit. Foss., p. 94, 1843. 
—  ostonca. G. B. Sowerby. Genera, No. 17, fig. 6. 
= — Woodward. Syn. Tab. Brit. Org. Rem., p. 15, 1830. 
a — S. Wood. Mag. Nat. Hist., New Series, vol. iv, p. 296, 1840. 
—  arotica. Brod. and Sow. Zool. Journ., No. xv, p. 359, t. ix, fig. 1, 1829. 
_ — Middendorff. Mem. de l’Acad. Imp. de St. Petersb. p. 544, 1849. 
Youpra Artica. Moller. Ind. Moll. Groen., p. 18, 1842. 
Spec. Char. Testa transversd, elongato-ovatd, inequilaterd, crassa ; anticé majiore et 
latiore elliptico-rotundatd, postice subrostratd ; eaterné striata, striis transversis obliquis, 
dentibus crassis angulatis. 
Shell transverse elongato-ovate, inequilateral, thick and strong, anterior side the 
larger and broader, elliptically rounded; posterior subrostrated ; externally striated, 
striz broad and oblique, teeth thick and angulated. 
Longitudinal diameter, 24 inches. y 
Locality. Red Crag, Bawdsey. 
Mam. Crag, Chillesford. Recent, Arctic Seas. 
