42 UNIONID^. 



sloping below : j^o^^^^'^*^'^ ^"^^ gradually sloping and com- 

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

 gaping : lower maryin 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 coloui', 

 which are sometimes alternate : heahs placed at a distance of 

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

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



Yar. 2. incrassata. Shell more swollen and solid, olive - 

 brown: wp^^er marc/in, or hinge-line, rather curved on the 

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

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



Var. 3. Zellemis. 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 curved, 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 resembling 

 in shape Modiola vulgaris : upper margin 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) Rossmassler, 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.paludosus 

 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 ; 



