416 Dr. J. E. Gray on some Families of Bivalve Shells. 



of the foot. Foot cylindrical, elongate, inferior, not byssiferous ; 

 lips none. Mantle doubled- edged, inner dilatile. 



a. Normal. *Lucina, Thyasira, Fimbria. **Codakia, Loripes. 



b. Gills two on each side, lips moderate. ?Ungulina. 



Suborder 2. Submytilacea. Foot large, compressed, anterior 

 adductor muscle nearly as large as the hinder. 



A. Shell free. Periostraca brown, hairy. Marine. 



2. Carditida, Syn. B. M. 1842, 79 ; P. Z. S. 1847, 193. Foot 

 compressed, conical, rather angulated behind. Cartilage external, 

 marginal. Hinge-teeth diverging, very oblique. Gills two pairs, 

 free, behind the body. 



3. Crassatellida, Syn. B. M. 1840, 141 ; 1842, 91 ; P. Z. S. 

 1847, 194. Gills two pair, dependent, produced, acute, and 

 united behind the base of the foot. Lips four, rhombic. Foot 

 short, compressed, triangular, with a deep groove. Mantle lobes 

 free, hinder end bearded, anal opening separate. Cartilage in- 

 ternal, in a triangular pit. Hinge-teeth diverging. 



B. Shell free. Periostraca hard, smooth. Fluviatile, 



4. Unionida, Syn. B. M. 1842, 80; P. Z. S. 1847, 196. Foot 

 compressed, subquadrate, angulated behind. Mantle edge 

 smooth, generally bearded behind. Gills two pair, large, 

 dependent. Shell pearly within; hinge variable, toothed or 

 toothless. Periostraca hard, polished. 



a. Unionina. Foot moderate. 



b. Mycetopedina. Foot elongate, enlarged at the end. Shell gaping 



at each end. Mycetopidse, Syn. B. M. 1842, 81 : P. Z. S. 

 1847, 197. 



C. Shell attached by the outer surface of one valve. Fluviatile. 



5. Etheriada, Syn. B. M. 1842,79; P. Z. S. 1847,193. Mantle 

 edge bearded. Gills large, two pair. Foot moderate, anterior 

 adductor muscle small, linear. Gills two pair, dependent. Shell 

 pearly and blistered within. Hinge toothless. Cartilage sub- 

 internal, cuiTcd. Young shell free, like Unio ? 



Suborder 3. Mytilacea. Foot small, ligulate, byssiferous; an- 

 terior adductor muscle small. 



6. Mytilad<E, Syn.B. M. 1842, 82 ; P. Z. S. 1847, 198. Anus 

 simple. Gills two on each side, dependent. 



