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 Nucula, the type of which is Area 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). 



Moller divided these latter or bilateral Nuculae into two genera, without, however, 

 any apparent distinction, either in the shell or animal ; Nucula arctica, the species he 

 intended as the type of his genus Yoldia, being furnished with a sinuated impression 

 like that of N. minuta (the typical form of Leda), 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. 



1. LEDA LANCEOLATA, /. Sowerby. Tab. X, fig. 16, a b. 



NUCULA LANCEOLATA. J. Sow. Min. Conch., t. 180, fig. 1, 1817. 

 Morris. Cat. of Brit. Foss., p. 94, 1843. 



OBLONGA. G. B. Sowerby, Genera, No. 17, fig. 6. 



Woodward. Syn. Tab. Brit. Org. Rem., p. 15, 1830. 



S. Wood. Mag. Nat. Hist., New Series, vol. iv, p. 296, 1840. 



ARCTICA. Brod. and Sow. Zool. Journ., No. XT, p. 359, t. ix, fig. 1, 1829. 



Middendorff. Mem. de 1'Acad. Imp. de St. Petersb. p. 544, 1849. 

 YOLDIA ARTICA. Moller. Ind. Moll. Groen., p. 18, 1842. 



Spec. Ckar. Testa transversd, elongato-ovatd, incequilaterd, crassd ; antice majiore et 

 latiore elliptico-rotundatd, postice subrostratd ; externe striatd, 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, 

 striae broad and oblique, teeth thick and angulated. 



Longitudinal diameter, 2j inches. 



Locality. Red Crag, Bawdsey. 



Mam. Crag, Chillesford. Recent, Arctic Seas. 



