CONCHIFERA. 305 



Animal with mantle-margins fringed; siphons unequal, more or less 

 separate ; branchial orifice sometimes doubly fringed, the outer pinnate ; 

 anal orifice with a simple fringe and tubular valve ; foot tongue-shaped ; palpi 

 small, lanceolate. 



V. textilis, and other elongated species, have a deep pallial sinus ; V. 

 gemma (Totten) has a very deep angular sinus, like Arlemis ; V. reticulata 

 has bifid teeth, like Tapes ; V. tridacnoides, a fossil of the U. States, has 

 massive valves, ribbed like the clam-shell. The N. American Indians used 

 to make coinage (wampum) of the sea-worn fragments of Venus mercenaria, 

 by perforating and stringing them on leather thongs. 



Distr. 176 sp. World-wide. Low-water 140 fathoms. V. astar- 

 toides, Behrings' Sea. V. verrucosa, Brit. Medit. Senegal, Cape, Red Sea; 

 Australia ? 



Fossil, 160 sp. Oolites . Patagonia, U. S. Europe, India. 



? Volupia rugosa, (Defrance, 1829.) Shell minute, Isocardia-shaped, 

 concentrically ribbed, with a large luimle. Eocene, Hauteville. 



Saxidomus (Nuttalli) Conrad. Oval, solid, with tumid umbones ; lunule, 

 ; teeth 3 4, unequal, the central bifid ; pallial sinus large. Distr. 8 sp. 

 India, Australia, W. America. 



CYTHEREA, Lam. 



Etym. Cytherea, from Cythera, an Aegean Island. 



Syn. Meretrix, Gray. Dione, Megerle. 



Examples, C. dione, PL XX. fig. 8. C. chione, fig. 14, p. 26. 



Shell like Venus ; margins simple ; hinge with 3 cardinal teeth and an 

 anterior tooth beneath the lunule ; pallial sinus moderate, angular. 



Animal with plain mantle-margins ; siphons united half-way. 



Distr, Same as Venus. Recent 113 sp. Fossil, 80 sp. 

 MERGE, Schum. 



Etym,. Meroe, an island of the Nile. 



Syn. Cuneus (part) Megerle (not Da Costa). Sunetta, Link. 



Type, M. picta (= Venus Meroe, L. Donax, Desh.) PI. XX. fig. 9. 



Shell oval, compressed ; anterior side rather longest ; hinge with 3 car- 

 dinal teeth, and a long narrow anterior tooth ; lunule lanceolate ; ligament 

 in a deep escutcheon. 



Distr. 1 sp. Senegal, India, Japan, Australia. 

 TRIGONA, Miihlfeldt. 



Etym. Trigonos, theee- cornered. Type, T. tripla, PI. XX. fig. 10. 



Shell trigonal, wedge-shaped, sub-equilateral ; ligament short, prominent ; 

 cardinal teeth 3 4, anterior f remote ; pallial sinus rounded, horizontal. 



Distr. 28 sp. W. Indies, Medit, Senegal, Cape, India, W. America. 



Fossil, Miocene . Bordeaux. 



T. crassatelloides attains a diameter of 5 inches and is very ponderous. 



P 2 



