CLASS I 



PELECYPODA 



467 



Periploma Scliuin. Shell oval oi' rounded, smootli or with i'aint concentric striae ; 

 lithodesma present. Tertiary and Recent. 



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

 desnia. Pliocene and Recent. 



Bontaea Leach {Ligula p.jj. Mont.) ; Tyleria Adams. Recent. 



Family 6. Thraciidae Dall. 



Shell earthy and cellulo-crystalline, not nacreous ; ineqioivalve, thin, edentidous, often 

 with a granular sihrface ; ligament and resilium ^PM 



chiefly external, opisthodetic, parivincular, seated 

 on posteriorly directed nymphae ; area absent, 



beaks usiudly entire ; valves nearly closed, with .^^!ft^\IJi0?!MK^S: // 



pallial sinus ; mantle openings small ; siphons 



long, sepjarated to their tips, naked ; monoecious : HfHiMPX " ^^^SS^^r^- /// 



marine. Jura to Recent. 



Thracia Leach {Gorimya Ag.) (Fig. 745). 

 Shell smooth or concentrically striated, with 

 granular surface, usually more or less rostrate. 

 Trias to Recent. 



Gyathodonta Conr. Shell witli oblique or 

 angular waves of sculjiture, otherwise like the 



preceding. Tertiary and Recent. 



Fid. 745. 



Thracia liuxrta Ag. Upptu- Jura ; Pruntiut, 

 SwitzerUuicl. i/j. 



B'ushia Dall ; Asthenothaerus Carpenter. Recent. 



Family 7. Myochamidae Dall. 



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

 margins of one valve overlapfing those of the other, which fit into corresponding dc])res- 

 sions in the shell loall ; 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 subgenus Kennerleya has a lithodesma. Tertiary and Recent. 



Goelodon Carp. Crura of the left valve united by a transverse lamella. Glidio- 

 phora Carp, has the hinge plate buttressed and a lithodesma. Tertiary and Recent. 



