THE LAMELLIBRANCHIA 273 



of the Chamacea, and possibly derived from the Megalodontidae of the 

 Palaeozoic and the Trias, has given rise to a branch that has survived to 

 the present epoch (Chamidae) and to various others that became extinct 

 at the close of the Secondary period. In all cases, the forms in which 

 the umbo of the free valve is coiled have preceded the forms with an 

 operculiform free valve Eequienia being derived from Diceras and 

 Cliama from Matheronia ; in the same way among the Rudistae Radiolites 

 appears to be derived from Caprina. The Hippuritidae, by the depth of 

 the fixed valve, the reduction of the cavity, and the absence of the 

 ligament, indicate the last stage of the evolutionary series. 



SUB-ORDER 7. MYACEA. 



Eulamellibranchia in which the mantle is closed to a considerable 

 extent ; the siphons are well developed, the gills much folded and 

 frequently prolonged into the branchial siphon. The foot is compressed 

 and generally byssif'erous. The shell gaping, with a pallial sinus. 



FAMILY 1. PSAMMOBIIDAE, Gray. Siphons very long and quite 

 separate. Foot large, flattened from side to side and pointed. Shell 



FIG. 245. 



Psanvmobia florida, right side, showing expanded foot (e) 

 and g, branchial, and g', anal siphons. (From Lankester, 

 after Garner.) 



oval, elongated, with a deep pallial sinus and an external ligament. 

 Genera Psammobia, Lamarck ; the posterior end of the shell sub- 

 truncated ; British (Fig. 245). Sanguinolaria, Lamarck. Asaphis, 

 Modeer. Elizia, Gray. Solenotellina, de Blainville. FAMILY 2. 

 MYIDAE, Gray. Mantle largely closed ; siphons united for the greater 

 part of their length and surrounded, near their extremities, by a circlet 

 of tentacles. Foot reduced. Shell gaping, with an internal ligament ; 

 the left valve provided with a spoon-shaped projection for the ligament. 

 Genera Mya, Linnaeus ; siphons elongated, covered by a chitinous 

 sheath, and incompletely retractile ; foot small ; palps elongated ; British. 

 Sphenia, Turton ; British. Tugonia, Gray. Platyodon, Conrad. Crypto- 

 mya, Conrad. FAMILY 3. CORBULIDAE, Fleming. Shell sub-trigonal, 

 inequivalve, the left valve less convex than the right ; the pallial sinus 

 shallow ; the ligament partly external. Siphons short, united, com- 

 pletely retractile. Foot large, pointed, often byssiferous. Palps reduced. 

 Genera Corbula, Bruguio.re ; siphons surrounded by a common circlet of 

 tentacles ; shell short ; British. Corlulomya, Nyst ; shell elongated ; 

 branchial siphon with a special tentacular crown. Paramya, Conrad. 

 Erodona, Daudin, and Himella, Adams, are fluviatile forms from South 

 America. FAMILY 4. LUTRARIIDAE, Adams. Mantle extensively 



18 



