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— Requienia being derived from Diceras and 
Chama 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, 
Sus-OrpER 7. Myacra, 
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 byssiferous. The shell gaping, with a pallial sinus. 
Famity 1. PsaMMositpan, Gray. Siphons very long and quite 
separate. Foot large, flattened from side to side and pointed. Shell 
Fig. 245. 
Psammobia 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).  Sanguwinolaria, Lamarck.  Asaphis, 
Modeer. Hlizia, Gray.  Solenotellina, de Blainville. Faminy 2. 
Mymak, 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. Famity 3. Corpunmpar, 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, Bruguiere ; siphons surrounded by a common circlet of 
tentacles ; shell short; British. Corbulomya, Nyst; shell elongated ; 
-branchial siphon with a special tentacular crown. Paramya, Conrad. 
Erodona, Daudin, and Himella, Adams, are fluviatile forms from South 
America. Famiry 4. lLurrarimpar, Adams. Mantle extensively 
18 
