2 7 6 



THE LAMELLIBRANCHIA 



pallial sinus. Genera Pholadomya, Sowerby ; some species living and 

 abyssal ; numerous fossil species from the Trias onwards, the maximum 

 in the Jurassic. FAMILY 5. ARCOMYIDAE, Fischer. Shell finely granular, 

 equivalve, thin ; the hinge without teeth ; the ligament external ; pallial 

 sinus. An exclusively fossil family, from the Secondary and Tertiary. 

 Genera Arcomya, Agassiz ; from the Trias to the Eocene. Goniomya, 

 Agassiz ; Jurassic and Cretaceous. FAMILY 6. PHOLADELLIDAE, Miller. 

 Shell oval, the posterior extremity attenuated and gaping ; cardinal 

 border thin and devoid of teeth ; ligament external ; posterior adductor 

 muscle large. An exclusively fossil family, from Primary 

 deposits. Genera Pholadella, Hall ; Devonian. l j hytimya, 

 Ulrich ; Silurian. Allorisma, King ; Carboniferous and 

 Permian. FAMILY 7. PLEUKOMYIDAE, Zittel. Shell inequi- 

 lateral, thin ; the pallial line deeply sinuous ; the cardinal 

 border of one valve covering that of the other and hiding 

 the ligament, which is therefore sub -internal. An ex- 

 clusively fossil family from Secondary formations. Genera 

 Pleuromya, Agassiz ; from the Trias and inferior 

 Cretaceous. Gresslya, Agassiz ; Jurassic. Ceromya, Agassiz ; 

 Jurassic. FAMILY 8. PANDORIDAE, Gray. Shell thin, 

 inequivalve, free ; the ligament internal ; no pallial sinus. 

 Siphons very short; foot elongate. Genera Pandora, 

 Bruguicre ; British. Coelodon, Carpenter. Clidiophora, 



Carpenter. FAMILY 9. MYO- 

 CHAMIDAE, Dall. Shell very 

 inequivalve, solid, with a 

 pallial sinus. Siphons short ; 

 a fourth pallial orifice 

 present ; foot small. Genera 

 Myochama, Stutchbury ; 

 shell irregular ; fixed to 

 other shells by the right 

 valve ; Australian. Myodora, 

 Gray ; shell free, trigonal ; 

 the left valve flattened. 

 FAMILY 10. CHAMOSTREI- 

 DAE, Fischer. Mantle 

 largely closed. A fourth 

 pallial orifice present ; pedal 

 orifice small. Siphons very 

 short and separate. Shell 



Atpergittum vaginiferum ; to the left side, the whole fi YP( l 1 1V fh p T \ a ]\t valvp irrp- 

 shell, dorsal view/the anterior part below ; to the right nxed DV . tJ ve > ir 



side, the anterior part, magnified, to show the original gular, Without a pallial Sinus ; 

 valves a, now embedded in a continuous calcification of ,. __._,. .'-.^-....i n an *, 

 tubular form. (From Lankester, after Owen.) ligament internal. Uenus 



Chamostrea, de Roissy ; 



Australian. FAMILY 11. CLAVAQELLIDAE, d'Orbigny. Mantle largely 

 closed ; pedal orifice extremely small ; a fourth pallial orifice present ; 

 siphons fairly long, united ; foot very rudimentary and without a byssus. 

 The ligament external ; the valves continued backwards into a calcareous 



Fio. 248. 



