CO^CRYLIA—DITHYRA. 45. 203 



Shell fixed, inequi valve, irregular ; with the beaks more or less se- 

 parated from each other and becoming unequal by age. Hinge 

 without teeth, with a cavity which is partly external and in the 

 lower valve becomes gradually elongated. Ligament internal. 



This well-known tribe is always parasitical, being fixed to rocks 

 and other marine substances by their under valve. The individuals 

 or species are subject to much irregularity of figure, and are gene- 

 rally coarse and rough. The hinge of the under valve becomes much 

 elongated by age, when the points of the beaks recede from each 

 other, and leave a gap between them. Most of them have some 

 minute granular denticulations on each side about the hinge. 



OsTREA testa suborbiculari foliaceo-squamosd trigono-plicatd, margine Crista Galli 



interno crenulato. ^• 



Shell somewhat orbicular with scaly foliations and triangular plaits, 



and the inner margin crenulate. 

 Mytilus Crista Galli Linn. Syst. Nat. p. 1155. 



Gmelin, Syst, p. 3350. 



Turton, Linn. Syst. iv. p. 289. 



Pennant, Brit. Zool, iv. p. 235. 



Montagu, Test. Brit. p. 166. 



Linn. Trans, viii. p. 104. 



Turton, British Fauna, p. 164. 



Dillwyn, Descript. Catal. p. 299. 



Turton, Conch. Diet. p. 109. 



Chemnitz, viii. p. 52, tab. 75, fig. 683, 684. 

 2d 2 



