102 MOLLUSCA FROM THE CRAG. 



These animals are bisexual, and the young shell is completely formed before 

 exclusion, although differing then materially in shape from that of its parent. Their 

 power of multiplication is said to be enormous. Mr. Lea states that he counted not 

 less than six hundred thousand young in an adult specimen. 



The shells of this genus are also exceedingly variable, more especially in the out- 

 ward form; some species have the hinge area largely elevated into the form of a wing, 

 and in consequence of these variations, as well as from differences in size of dental 

 characters, they have also been separated into several genera. 



It appears to be a modern genus, and only yet known in the fossil state from the 

 newer Tertiaries. 



1. ANODONTA CYGNEA, Linneeus. Tab. XI, fig. 11. 



MYTILUS CYGNEUS. Linn. Syst. Nat., ed. 12, No. 257, 1158, 1767. 



Mat. and Rack. Linn. Trans., vol. viii, pi. 3A, fig. 2, 1807. 

 Poli. Test. Sicil., vol. ii, p. 212, pi. 33, fig. 2, 1795. 

 Sheppard. Trans. Linn. Soc., vol. xiii, p. 84, pi. 5, fig. 3, 1822. 



ANATINUS. Linn. Syst. Nat., ed. 12, p. 1158. 



Poli. Test. Sicil., vol. ii, p. 213, pi. 33, fig. 1, 1795. 



AVONENSIS. Mont. Test. Brit., p. 172, 1803. 



Mat. and Rack. Linn. Trans., vol. viii, p. 250, pi. 3. 



MACULA. Sheppard. Linn. Trans., vol. xiii, p. 88, pi. 5, fig. 6, 1822. 

 STAGNALIS. Sowerby's Brit. Miscellany, pi. 16. 



Brown. Illust. Brit. Conch., pi. 27, fig. 2, 1827. 

 DENTATUS. Turt. Conch. Diet., p. 115, 1819. 



INCRASSATUS. Sheppard. Linn. Trans., vol. xiii, p. 85. pi. 5, fig. 4. 

 ANODONTA CYGNEA. Pfeiff. Land und Sussw. Moll., p. Ill, t. vi, fig. 4, 1821. 



ANATINA. Id. - p. 112, t. vi, fig. 2, 



INTERMEDIA. Id. - p. 113, t. vi, fig. 3, 



VENTRICOSA. Kickx. Moll. Brab. Aust., p. 80. 



PISCINALIS. Nils. Moll. Succ. Ter. et Fluv., p. 116, fide Forb. and Hani. 



CYGNEA. Rossm. Icon. Land and Sussw. Moll., pp. 1, 111, t. 3, fig. 67, 1835. 

 ANODON PALUDOSA. Turt. Brit. Biv., p. 240, pi. 15, fig. 6, 1822. 

 SYMPHYNOTA CYGNEA. Lea. Obs. on the Gen. UNIO, vol. i, p. 70, 1832. 



Spec. Char. Testa oblongo-ovata, scepe compressd, tenui, interdum tumidd et incrassafd ; 

 antice rotundatd, postice productd, et angulatd ; natibus depressiusculis, rugosis. 



Shell oblongo-ovate, generally compressed and thin, sometimes tumid or inflated, 

 occasionally thick ; anterior side rounded, posterior produced, and angulated ; umbones 

 rather flat, with undulating rugosities. 



Length, 3^ inches. Height, 2 inches. 



Locality. Stutton, Clacton. Grays (Pickering] , Cropthorn, and Bacton (Morris}. 



Recent, Britain, and North of Europe. 



This species is abundant in individuals both at Stutton and Clacton, the two 

 localities of Fresh-water Deposits that I am best acquainted with, although from 

 their great fragility specimens are very difficult to obtain in any degree of 



