276 THE LAMELLIBRANCHIA 
pallial sinus. Genera—Pholadomya, Sowerby ; some species living and 
abyssal ; numerous fossil species from the Trias onwards, the maximum 
in the Jurassic. Famity 5. ArcomyrDAg, 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. Phytimya, 
Ulrich ; Silurian. Allorisma, King; Carboniferous and 
Permian. Famity 7. PLEUROMYIDAE, 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. Faminy 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. Famity 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. 
Famity 10. CHAMOSTREI- 
DAE, Fischer. Mantle 
largely closed. A fourth 
pallial orifice present ; pedal 
orifice small. Siphons very 
short and separate. Shell 
Aspergillum vaginiferum ; to the left side, the whole ye = as oe . 
shell dowel view, the anterior part below; to the right fixed by the right valy e, irre 
side, the anterior part, magnified, to show ne original gular, without a pallial sinus ; 
valves a, now embedded in a continuous calcification of 17- : 
tubular form. (From Lankester, after Owen.) ligament internal. Genus— 
Chamostrea, de Roissy ; 
Australian. Famiby 11. CLAVAGELLIDAE, 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 
y 
hy Wh Mca 
STL Ta | 
Fic. 248. 
