ANODONTA. 157 



but even demur to the possibility of arranging the diversi- 

 ties of shape and colouring into strictly defined varieties, so 

 imperceptibly does one form glide into another, scarcely 

 indicating by any preponderance of peculiarities, under 

 which heading it should be ranked. Still there are certain 

 individuals so strongly characterised, that selecting them 

 as centres we may group around them the majority of ex- 

 amples, leaving, nevertheless, many specimens remaining, 

 which, except by infinite subdivision of nomenclature, and 

 the objectionable formation of subvarieties, cannot well be 

 classified, but oscillate between the more marked variations. 

 The typical form of the primitive cygnea {Mytilus cygneus, 

 ScHROTER, Flussconch. pi. 3, f. 1. — Pennant. Brit. Zool. 

 ed. 4, vol. iv. p. 113, pi. 67, f 78.— Donov. Brit. Shells, 

 vol. ii. pi. 55. — Maton and Rack. Linn. Trans, vol. viii. 

 pi. 3a. f. 2. — Index Testaceolog. pi. 12, Myt. f. 32. — 

 Myt. stagnalis, Gmelin, Syst. Nat. p. 3362, from Schroter 

 Flussconch. pi. 1. f. 1. — Myt. anatinus var. h. Sturm, 

 Deutsch. Fauna, pt. 1. — Anodonta cygnea, Brard, Ooq. de 

 Paris, pi. 10. — Pfeiffer, Deutsch. Land und Siissw. Moll, 

 pt. 1, p. Ill, pi. 6, f. 4, and pt. 2, pi. 6. — Rossmas. Iconog. 

 Land und Siissw. Moll. pt. 1, p. Ill, pi. 3, f. 67, and pt. 5, 

 pi. 25, f. 342. — Brown, 111. Conch. G. B. pi. 28, f. 1) is 

 oval or elongated oval, moderately and not very unevenly 

 ventricose, and but little oblique or solid. The anterior 

 side, which does not quite occupy one-third of the length is 

 broadly rounded (occasionally angulated above) ; the pos- 

 terior end is attenuatedly rounded, but not beaked, and 

 devoid of peculiar retusion at the npper edge. From the 

 dorsal and ventral margins running nearly parallel to each 

 other (the latter is rather convex, the former straight or 

 slightly swelling and not rising into a wing), the two extre- 

 mities of the shell are not so unequal as in several of the other 



