CORBULTD^. 139 



BOTHROCORBULA, Gabb, 18*72. Differs from the typical Corbulae 

 in having a deep Innular pit under the beaks, penetrating and 

 almost passing through the hinge-plate. C. viminea, Guppy 

 (cvii, 36, 31). 



PTEROMYA, Moore. Resembles Corbula, but thin ; smooth or 

 concentrically striated. Ft. Growcombei^ Moore. Rhsetic beds, 

 at Beei'-Crowcombe. 



HiMELi^^., H. Adams, 1860. Shell thin, with the left valve 

 larger than the right one, not gaping ; hinge of the right valve 

 with an indistinct tooth fitting into a pit in the left valve ; 

 cartilage internal, lying in both valves in an almost horizontally 

 extending process ; an external ligament is besides present ; 

 pallial sinus scarcely noticeable. Based on H. Jluviatilis, Mara- 

 non Riv. 



POTAMOMYA, J. Sowerby, 1839. (Azara, d'Orbigny, 1839.) P. 

 gregaria. Eocene ; Isle of Wight. Cartilage-process broad and 

 spatulate, received between two obscure teeth in the right valve. 

 The estuary Corbulse differ very little from the marine species. 

 G. labiata lives buried in the mud of the River Plata, but not 

 above Buenos Ayres, and consequently in water which is very 

 little influenced by the superficial ebb of the river. The same 

 species is found in banks widely dispersed over the Pampas near 

 San Pedro, and many places in the Argentine Republic, five 

 yards above the River Parana. (Darwin.) C. erodina^ Lara. 

 (cvi, 23-25). 



CORBULAMELLA, Meek and Worthen, 1851. Shell subtriangular, 

 subglobose, inequivalve, the right valve being more ventricose 

 than the left ; beaks nearly central ; hinge with one cardinal 

 tooth in each valve, apparently very similarly arranged in position 

 to that of Corbula, but the existence of an internal cartilage has 

 not as yet been satisfactorily proved ; anterior muscular impres- 

 sion rather indistinct, posterior on a special raised or projecting 

 plate; pallial impression scarcely sinuated posteriorly. G. gre- 

 garia^ M. and H. (cv, 95). Cretaceous ; Nebraska. 



Plectodon, Carpenter, 1865. 



Distr. — P. sca,ber, Carp. Catalina Isl., Cal. 



Shell thin, rough, rostrate ; dorsal margin twisted within under 

 the umbones, forming the cardinal tooth ; lateral teeth long, 

 laminated ; cartilage-pit minute, concealed under the umbones ; 

 posterior lateral tooth contiguous ; pallial sinus small. 



Has the aspect of Theora, and appears allied to Nesera. It is 

 probable that the cartilage was strengthened by an ossicle. The 

 great peculiarity is the twisting-in of the dorsal margin, which 

 ascends the umbo in a very loose spiral. 



Sphenia, Turton, 1822. 

 Distr. — 4 sp, Britain, W. Coast of N. Am., Red Sea. Bur- 



