390 MOLLUSCA SUB-KINGDOM vi 



broad and shallow ; siphons separated to their bases, naked and wholly retractile ; mono- 

 ecious ; marine. Tertiary and Kecent. 



Periploma, Sclium. Shell oval or rounded, smooth or with faint concentric striae ; 

 lithodesma present. Tertiary and Recent. 



Cochlodesma, Couth. Buttress of the chondrophore posteriorly directed ; no litho- 

 desma. Pliocene and Recent. 



Bontaea, Leach (Ligula, p.p. Mont.) ; Tyleria, Adams. Recent. 



Family 6. Thraciidae. Dall. 



Shell earthy and cellulo-crystalline, not nacreous ; inequivalve, thin, edentulous, often 



with a granular surface ; ligament and resilium 

 chiefly external, opisthodetic, parivincular, seated 

 on posteriorly directed nymphae ; area absent, 

 beaks usually entire; valves nearly closed, with 

 pallial sinus; mantle openings small; siphons 

 long, separated to their tips, naked; monoecious; 

 marine. Jura to Recent. 



Thracia, Leach (Corimya, Ag.), (Fig. 696). 

 Shell smooth or concentrically striated, with 

 granular surface, usually more or less rostrate. 

 Trias to Recent. 



Fl0 - 696 ' Cyathodonta, Conr. Shell with oblique or 



8 - u PP er Juri ' Pruntrut ' angular waves of sculpture, otherwise like the 



preceding. Tertiary and Recent. 

 Bushia, Dall ; Asthenothaerus, Carpenter. Recent. 



Family 7. Myochamidae. Dall. 



Shell very ineqidvalve, free or sessile, solid, sub-nacreous, edentulous, the dorsal 

 margins of one valve overlapping those of the other, which jit into corresponding depres- 

 sions in the shell wall; ligament amphidetic, external or absent; resilium internal, 

 alivincular ; area amphidetic or obsolete, a false area formed on each side of the beaks by 

 the flattened cardinal margin of the valves ; shell closed ; pallial sinus small. Tertiary 

 and Recent. 



The gills and siphons of Myochama, Stutchbury, which lives sessile on shells, are 

 more like those of Thracia than of the Pandoridae, with which it has usually been 

 associated. The anatomy of Myodora is unknown. Its minute area curiously recalls 

 that of Spondylus, and it is free. 



SECTION B. ADELOSIPHONIA. 



Family 8. Pandoridae. Gray. 



Shell compressed, inequivalve, free, solid, with nacreous and prismatic layers; the 

 dorsal edges of the valves overlapping, but not socketed, ivith dentiform crural ridges on 

 either side of the resilium, but no true teeth; ligament amphidetic, external, obsolete; 

 resilium internal, opisthodetic, usually reinforced on its anterior surface by a mesial 

 elongate lithodesma ; area none ; valves closed, beaks entire, pallial line simple ; marine. 

 Cretaceous to Recent. 



"' Pandora, Brug. Diverging crura without connecting lamellae ; buttress and 

 lithodesma absent. The sub-genus Kennerlia has a lithodesma. Tertiary and Recent. 



