MOLLUSCA : LAMELLIBRANCHIATA. 327 



Two subdivisions are comprised in this section. In the 

 first the siphons are short, and the pallial line is simple (Integro- 

 pallialia) ; as is seen in the families Chamidcz, Hippuritidce, 

 Tridacnidce, Cardiadtz, Lucinida, Cycladida, and Cyprinidce. 



The second subdivision (Sinu-pallialia) is distinguished by 

 the possession of long respiratory siphons, and a sinuated pallial 

 line, and it comprises the families Veneridce, Mactridce, Tellinida, 

 Solenidtz, Myaddce, Anatinidce, Gastrochcznidce, and Pholadida. 



SYNOPSIS OF THE FAMILIES OF THE LAMELLIBRANCHIATA. 

 SECTION A. ASIPHONIDA. 



Fam. I. Ostreidce. Shell inequivalve, slightly inequilateral, free or 

 attached ; hinge usually edentulous. Ligament internal. Lobes 

 of the mantle entirely separated ; the foot small and byssiferous, 

 or wanting. A single adductor. 111. Gen. Ostrea, Pecten, Spon- 

 dylus, &>c. 



Fam. 2. Aviculidcz. Shell inequivalve, very oblique, attached by 

 a byssus ; hinge nearly, or quite, edentulous. Mantle-lobes free ; 

 anterior adductor small, leaving its impression within the umbo ; 

 posterior adductor large and sub- central. Foot small. 111. Gen. 

 Avicula, Inoceramus, Pinna. 



Fam. 3. Mytilidce. Shell equivalve, umbones anterior, hinge eden- 

 tulous ; anterior muscular impression small, posterior large. 

 Shell attached by a byssus. Mantle-lobes united between the 

 siphonal apertures. Foot cylindrical, grooved, and byssiferous. 

 111. Gen. Mytilus, Modiola, Dreissena. 



Fam. 4. Arcades. Shell equivalve ; hinge long, with many comb- 

 like equal teeth. Muscular impressions nearly equal. Mantle- 

 lobes separated ; foot large, bent, and deeply grooved. 111. Gen. 

 Area, Pectunculus, Cucullcea. 



Fam. 5. Trigoniadce. Shell equivalve trigonal; hinge-teeth few, 

 diverging ; umbones directed posteriorly. Mantle open ; foot 

 long and bent. 111. Gen. Trigonia, Axinus. 



Fam. 6. Unionidce. Shell usually equivalve, with a large external 

 ligament. Anterior hinge-teeth thick and striated ; posterior 

 laminar, or wanting. Mantle-lobes united between the siphonal 

 apertures. Foot very large, compressed, byssiferous in the fry, 

 111. Gen. Unio, Anodon, Midler ia. 



SECTION B. SIPHONIDA. 



Subdivision I. Integro-pallialia. Siphons short, pallial line 

 simple. 



Fam. 7. Chamidce. Shell inequivalve, attached ; hinge-teeth 2-1 

 (two in one valve and one in the other). Adductor impressions 

 large. Mantle closed ; pedal and siphonal orifices small and 

 nearly equal. Foot very small. 111. Gen. Chama, Diceras. 



Fam. 8. Hippuritidce. " Shell inequivalve, unsymmetrical, thick, 

 attached by the right umbo ; umbones frequently camerated ; 

 structure and sculpturing of valves dissimilar ; ligament internal ; 

 hinge-teeth 1-2 ; adductor impressions 2, large, those of the left 

 valve on prominent apophyses ; pallial linefsimple, sub-marginal " 

 (Woodward). 111. Gen. Hippurites, Radiolites, Caprinella. 



