390 MOLLUSCA SUB-KINGDOM VI 
broad and shallow ; siphons separated to their bases, naked and wholly retractile ; mono- 
ectous ; marine. Tertiary and Recent. 
Periploma, Schum. 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. 
Ae Cyathodonta, Conr. Shell with oblique or 
Thracia incerta, Ag. Upper Jura; Pruntrut, i gees te ys Pn . : 
Switzerland, 1/;, angular waves of sculpture, otherwise like the 
preceding. Tertiary and Recent. 
Bushia, Dall; Asthenothaerus, Carpenter. Recent. 

Fic. 696. 
Family 7. Myochamidae. Dall. 
Shell very inequivalve, free or sessile, solid, sub-nacreous, edentulous, the dorsal 
margins of one valve overlapping those of the other, which fit into corresponding depres- 
stons 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, with 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. 
