318 MANUAL OF THE MOLLUSCA. 



Distr. 50 sp. U. S. Norway, Brit. Medit. W. Africa, China. Inhabits 

 sandy bottoms ; Lower laminarian zone 80 fms. 



Fossil. 90 sp. Inf. Oolite . U. States, Europe, India. The external 

 shell- layer consists of fusiform cells ; the inner is homogeneous and adheres 

 so slightly to the outer layer, that it is very frequently detached in fossil 

 specimens. Corbulomya, Nyst (C. complanata, Sby.) Crag. Brit. 



Sub-genera. Potamomya, J. Sby. P. gregaria, Eocene, I. Wight. Car- 

 tilage process broad and spatulate, received between two obscure teeth in the 

 right valve. The estuary Corbula differ very little from the marine species. 

 P. labiata (Azara, D'Orb.) PI. XXIII. fig. 3, lives burie* in the mud of the 

 R. Plata, but not above Buenos Ayres, and consequently in water which is 

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

 found in banks widely dispersed over the Pampas near S. Pedro, and many 

 places in the Argentine Republic, 5 yards above the R. Parana. (Darwin.) 



Sphenia, Turt. S. Binghami, PI. XXIII. fig. 4. Shell oblong; right 

 valve with a curved, conic tooth in front of the oblique, sub -trigonal car- 

 tilage-pit. Animal with thick united siphons, fringed at the end, anal valve 

 conspicuous ; foot finger-like, with a byssal groove. Distr. Brit. France. 

 Burrowing in oyster-shells and limestone, in 10 25 fms. Fossil, Miocene . 

 Brit. 



NEJGRA, Gray. 



Etym. Neara, a Roman lady's name. 



Type, N. cuspidata, PL XXIII. fig. 5. Syn. Cuspidaria, Nardo. 



Shell globular, attenuated and gaping behind; right valve a little the 

 smallest ; umbones strengthened internally by a rib on the posterior side ; 

 cartilage process spatulate, in each valve, (furnished with a moveable ossicle, 

 Deshayes) with an obsolete tooth in front, and a posterior lateral tooth ; 

 pallial sinus very shallow. 



Animal with the mantle closed; foot lanceolate; siphons short, united, bran- 

 chial largest, anal with a membranous valve, both with a few lonsc, lateral cirri. 



Distr. 20 sp. Norway, Brit. Medit, Canaries, Madeira, China, Moluccas, 

 New Guinea, Chile. Prom 12200 fms. 



Fossil, 6 sp. Oolite . Brit. Belgium, Italy. 



Fig. 221. Thetis, minor, Sby. Neocomian, I. Wight. 



