INSTITUTE OF SCIENCE, PHILADELPHIA. 533 



chondrophoric submarginal ridge on each valve ; beaks entire, valves almost 

 closed, pallial sinus distinct. 



Tertiary to recent fauna. 

 Ex. Lyonsia, Mytilimeria, Entodesma, Endomargarus, Actmomya. 



A small Indian species of Lyonsia is said to live in fresh water. Many 

 of the species attach grains of sand to the exterior of the valves, and Mytili- 

 meria lives in a self-made spongy mass of solidified mucus and sand. Nearly 

 all are nestlers. 



Family LYONSIELLID^. 



Limbs of the branchiae spread out, attached to an infolded lamina, project- 

 ing from the tissues of the mantle, which forms part of the septum between the 

 anal and branchial chambers; palpi large, feebly striated; foot small, grooved, 

 byssiferous, with or without an opisthopodium, but in either case without an 

 opisthopodial foramen ; pedal foramen large ; siphons short, separate, the 

 branchial papillose, usually with a large and conspicuous curtain valve; monoe- 

 cious, marine. 



Shell nearly equivalve, subnacreous, with a more or less distinct tubercle 

 in front of the resilium on the dorsal margin ; ligament obsolete, cartilage in- 

 ternal with a large lithodesma ; area obscure or none ; beaks entire, valves 

 almost closed ; pallial sinus obsolete. 



Tertiary to recent fauna. 

 Ex. Lyonsiella, Halicardia. 



Halicardia has a finlike opisthopodium, which is absent in Lyonsiella' 

 The branchial septum is normally complete, though the stems of the gills are 

 not attached to the siphonal septum, but only the margin of the lamina ; the 

 attachment is very delicate and is often ruptured in well-preserved specimens 

 and, perhaps, even in living specimens. Owing to the large pedal foramen 

 there is no necessity for a fourth opening between the mantle to permit the 

 escape of water on sudden closing of the valves, and so it has disappeared. 



Family CHAMOSTREID^. 



Gills with the external direct lamina free, united behind ; foot small, com- 

 pressed, not byssiferous; opisthopodium wanting, but an opisthopodial fora- 

 men persists ; pedal foramen small ; siphons short, separate, papillose, branch- 

 ial with a curtain valve ; marine. 



Shell sessile, inequivalve, irregular, solid, subnacreous ; hinge with a tu- 

 bercle in front of the resilium in the right valve, fitting an equivalent socket 

 in the left valve; ligament obsolete; resilium strong, internal, submarginal, 

 reinforced by a large lithodesma; area none, valves close fitting, pallial line 

 simple. 



Recent in Austral seas. 

 Ex. Chaniostrea. 



