534 LECTURE XXI. 



closed ; cardinal teeth not exceeding two, laterals 1-1, sometimes 

 wanting. 



Genera Tellina, Psammohia, Sanguinolaria, Syndosmya, Scrohi- 

 cularia, Mesodesma, Donax. 



c. Mantle sheath-shaped, closed, save at the pedal and siphonal 



apertures. 



Family Solenidce. 



Siphons short and united (in the long-shelled genera), long and 

 partly separate (in the shorter and more compressed genera) ; gills 

 prolonged into the branchial siphon ; foot large^ subcylindrical. Shell 

 gaping at both ends. 



Genera Solen, Cultellus, Machoera, Solecurtus. 



Family MyacidcB. 



Siphons united ; foot small. Shell gaping behind. 

 Genera Mya, Corhula, Necera, Thetis, Panopcea, Saxicava, Gly- 

 cimeris. 



Family Anatinidce. ■ 



Siphons long, more or less united ; foot very small. Shell often 

 inequivalve. 



Genera Anatina, Thracia, Pholadomya, Lyonsia, Pandora, Myo- 

 charna, Chamostrea. 



Family Gastrochanidce. 



Siphons long, united to near the extremities ; foot finger-shaped, 

 sometimes grooved and byssiferous ; mantle with a boring disc in 

 front. Shell regular, wedge-shaped, gaping in front, in some more 

 or less cemented to a superadded calcareous tube. 



Genera Gastrochcena, Clavagella, Aspergillum. 



Family Pholadidce. 



Animal clavate or vermiform ; siphons large, long, united nearly 

 to their ends ; foot short and truncate. Shell gaping at both ends ; 

 hingeless, with sometimes accessory valves, or a supplementary tube 

 .and palettes. 



Genera Pholas, Xylophaga, Teredo, Teredina. 



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