160 UNIONIDiE. 



less compressed, rounded both above and below at the very 

 short anterior extremity, and produced posteriorly into a long 

 rostrum, whose subtruncated tip is generally rounded off. 

 Its dorsal line is arcuated, and slopes downwards in front ; 

 behind it sometimes rises, sometimes falls, and the upper 

 posterior edge is occasionally retuse, occasionally a little 

 convex : the ventral edge is straiohtish or incurved. The 

 epidermis seems almost always of a brownish or dusky 

 olive-colour, and this is likewise the case with Avonensis 

 {Mytilus Avonensis, Mont, Test. Brit. p. 172. — Turt. Conch. 

 Diction, p. 116. — Index Testaceolog. pi. 12, f. 34<. — Mytilus 

 anatinus, var. Avoiiensis, Maton and Rack. Linn. Trans, 

 vol. viii. pi. 8a. f. 4. — 3Iytilus incrassatus, Sheppard, Trans. 

 Linn. Soc. vol. xiii. pi. 5, f. 4. — Anodon Avonensis, Turt. 

 Dithyra Brit. p. 241. — Anodon cygneus var. Avonensis, 

 Brown, 111. Conch. G. B. pi. 29, f. 2), which is of a some- 

 what oval shape, rounded at both ends, short, and narrower 

 in front, where the dorsal edge manifestly declines, and 

 neither distinctly winged nor beaked behind, where the 

 dorsal line is generally a little arched, and the upper pos- 

 terior margin a little convex. The connection between 

 this and the preceding is very close, the absence of the 

 rostrum almost constituting the sole difference. 



The limits of our work forbid the specification of the less 

 striking variations. Dusky and solid examples, which 

 bear some resemblance to the last in colouring, and the fall 

 of the front dorsal edge, but are more rhomboidal and sub- 

 rostrated, are usually termed ponderosa iJSIyt. cygneus var. 

 Maton and Rack. Linn. Trans, vol. viii. pi. 3 a. f. o. — 

 RossM. Iconog. Land und Slissw. Moll. pt. 4, pi. 20, f. 282. 

 — Anodonta ponderosa, Pfkif. Deutsch. Land und Slissw. 

 Moll. pt. 2, pi. 4.— KicKx, Moll. Brab. Austr. p. 81) : the 

 paludosa of Turton is beautifully radiated with light yel- 



