CLASS I PELECYPODA 467 



Periploma Schiiin. Shell oval ov rounded, siiiootli or witli faiiit coucentric Striae ; 

 lithodesnia preseiit. Tertiaiy and Eecent. 



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

 desiiia. Pliocene and Recent. 



Bontaea Leach {Ligula p.p. Mont.) ; Tyleria Adams. Kecent, 



Family 6. Thraciidae Dali. 



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

 with a granulär surface ; licjament and resilium 

 chiefly external, opisthodeticj parivincular, seated 

 on posteriorly directed nymphae ; area absent, 

 heaJcs usually entire ; valves nearly closed, with 

 pallial sinus ; mantle openings small ; siphons 

 long, separated to their tips, nahed ; monoecious ; 

 marine. Jura to Recent. 



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

 Shell smooth or concentrically striated, with 

 granulär surface, usually more or less rostrate. 

 Trias to Recent. 



Gyathodonta Conr. Shell with oblique or ■^^^' '^'^^• 



angular waves of sculpture, otherwise like the ^'"««^ ^«<^ß»-^ Ag.^^Upper Jum 

 preceding. Tertiary and Recent. 



Bushia Dali ; Asthenothaerus Carpenter. Recent. 



Family 7. Myochamidae Dali. 



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

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

 sions in the shell wall; ligament amphidetic, external or ahsent ; 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 lias 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 socheted, 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 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. 



