42 UNIONID^. 



sloping below : posterior side gradually sloping and com- 

 pressed above, produced into a rounded wedge-like point, and 

 gaping : lower margin nearly straight : inside pearl-white and 

 highly iridescent : hinge slight, having a rather sharp ridge - 

 like plate on the posterior side in each valve : muscular and 

 pallial scars very slight and indistinct. L. 2*75. B. 5-35. 



Var. 1. radiata. Shell larger, yellowish-green, beautifully 

 marked with longitudinal rays or streaks of the same colour, 

 which are sometimes alternate : heaJcs placed at a distance of 

 only one-third from the anterior side. Mytilus radiatus, Miill. 

 Verm. Hist, pt. ii. p. 209. 



Var. 2. incrassata. Shell more swollen and solid, ohve- 

 brown : upper margin, or hinge-line, rather curved on the 

 posterior side. Mytilus incrassatus, Shepp. in Linn. Trans. 

 xiii. p. 85, pi. 5. f. 4. 



Var. 3. Zellensis. Shell broader, yellowish-brown, having 

 the upper and lower sides nearly parallel ; 'posterior side much 

 produced. Mytilus Zellensis, Gmel. Syst. Nat. p. 3262. 



Var. 4. pallida. Shell light yellow or fawn-colour : hinge- 

 line rather cui'ved, and raised on the posterior side, which is 

 produced to a long wedge-like point : lower margin rounded. 



Var. 5. rostrata. Shell oblong-oval, somewhat resembhng 

 in shape Modiola vulgaris : upper margiyi forming a dorsal 

 crest, which is slightly raised and curved: anterior side 

 rounded : posterior side attenuated, and ending in a long 

 curved wedge-Hke point : lower margin nearly straight. A. 

 rostrata, (Kokeil) Eossmiissler, Iconogr. iv. p. 25, f. 284. 



Habitat : Slow rivers, lakes, canals, and ponds 

 throughout the kingdom as far north as Banffshire ; and 

 it is one of our upper tertiary fossils. It ranges from 

 Siberia to the Pyrenees. Var. 1. Bog of Allen, Ireland 

 (Turton) ; Clumber lake, Notts (J. G. J.). This variety 

 is the Mytilus stagnalis of Gmelin, the M. dentatus of 

 Turton's Conchological Dictionary, and the M.jMludosus 

 of his work on the British Bivalves. Specimens of this 

 variety measure upwards of 6 inches in breadth. Dr. 

 Turton^s type (of which only one valve remains) has a 

 small pearly tubercle on the ridge of the laminar tooth ; 



