34 ANATlxVID^. 



continually exercised by the strong and elastic cartilage 

 exceeds that of the shell ; and the latter being the weaker 

 body, gives way and is split in the conflict. Only one 

 species [T. distort a), which is comparatively more solid 

 than the others, resists the strain and remains un- 

 injured. 



The synonymy of the European species has been 

 lamentably perplexed ever since the time of Pennant, 

 notwithstanding the pains taken by Loven and the 

 authors of the ' British Mollusca^ to unravel the tangled 

 skein. This makes it extremely difficult to define with 

 any certainty the geographical distribution of some of 

 these species. Geologically Thracia appears to be an 

 ancient genus. " Fossils of this form are found in the 

 lower Oolites, and doubtfully so in the Carboniferous 

 series" (S. Wood). It is the genus Odoncincta of Da 

 Costa, and has received other equally barbarous names 

 from modern authors. Cochlodesma, Couthouy, does 

 not difter in any respect except in the absence of an 

 ossicle : all the British species of Thracia possess this 

 appendage. 



A. Nearly equilateral. 

 1. Thracia PRiETE^NUis^^ Pulteney. 



Mya pratenuis, Pult. Cat. Dors. p. 28, pi. iv. f. 7. Cochlodesma prmtenue, 

 "r. & H. i. p. 235, pi. XV. f. 4. 



Body thin, clear white : gills strongly pectinated, eacli 

 divided by an oblique furrow into two parts, the upper being 

 less deep than the lower portion : foot white. 



Shell triangularly oval, compressed, opaque, somewhat 

 glossy ; right valve more convex than the left, and slightly 

 overlapping it: sculpture (besides the usual marks of growth), 

 close-set and microscopical transverse hair-like lines or scratches 



* Very tliin. 



