364 ANATINID^. 



nia, in sponge thrown up attached to the roots of fuci in 

 deep water; the other species, one of which is the Byssonia 

 cuneata of Valenciennes, are also found living in Ascidice. 

 Dr. Gray ohserves that he has discovered the mark left by 

 the shelly plate or ossicle over the large, sub-internal car- 

 tilage in M. Nuttallii. 



Species of Mytilimeria. 



cuneata, Valenc. Nuttallii, Com: 



navicula, Adams and Beeve. picta, Sow. 



Genus THRACIA, Leach. 



Siphons divergent, separate nearly their entire length, 

 their orifices fringed. 



Shell transversely ovate, thin, inequivalve, compressed, 

 more or less gaping posteriorly, surface of valves nearly 

 smooth, or minutely scabrous, beaks entire. Hinge com- 

 posed of a slightly-prominent cartilage-process in each 

 valve ; cartilage with a free, crescentic ossicle in front ; 

 ligament partly internal. Muscular impressions small and 

 dissimilar ; pallial line strongly sinuated. 



Syn. Odoncinetus, Costa. Odontocincta, Ayass. Odon- 

 cyneta, Cantr. Cinctodonta, Herrm. Osteodesma, Blainv., 

 not Desk. 



Ex. T. declivis, Pennant, pi. 96, fig. 5. Shell, T. 

 pubescens, Pultney, fig. 5, a, 5, b. 



M. Recluz, in separating the Biqncola of Bellevue as a 

 genus distinct from Thracia, observes that the foot is 

 simple, and that there is only a single gill, while in Thracia 

 he states that the foot is tri-lobed, and the gills two on each 



