256 MANUAL OF ZOOLOGY. 



The second subdivision (Sinu-pallialia) is distinguished 

 by the possession of long respiratory siphons, and a siuuatcd 

 jmllial line, and it comprises the families Veneridce, Mactridw, 

 Tellmidce, Solenidce, Myacidce, Anatinidce, Gastroclicenidce, and 

 Pholadidcv. 



SYNOPSIS OF THE FAMILIES OF THE LAMELLIBBANCHIATA. 



SECTION A. ASIPHONIDA. 



Fam. 1. Ostreida. Shell inequivalve, slightly inequilateral, free 

 or attached; hinge usually edentulous. Ligament internal. 

 Lobes of the mantle entirely separated ; the foot small and byssi- 

 ferous, or wanting. A single adductor. 111. Gen. Ostrea, Pectcn, 

 Spondylus, #c. 



Fam. 2. Aviculidee. 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. 

 III. Gen. Mytilus, Modiola, Dreissena. 



Fam. 4. Arcada. 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, Pectuncufus, Cucull&a. 



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. Unionida. 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, Mulleria. 

 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, un symmetrical, thick, 

 attached by the right umbo ; umbones frequently cam crated); 

 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 line simple, sub-marginal.' 

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



Fam. 9. Tridacnidcs. Shell equivalve ; ligament external ; muscular 

 impressions blended, sub-central. Animal attached by a byssus, 

 free. Mantle-lobes extensively united ; pedal aperture large ; 

 siphonal orifices surrounded by a thickened pallial border. Foot 

 finger-like and byssiferous. 111. Gen. Tridacna. 



Fam. 10. Cardiadce. Shell equivalve, heart-shaped, with radiating 

 ribs; cardinal teeth 2 ; lateral teeth 1-1, in each valve. Mantle 



