532 • LECTURE XXI. 



Family Arcadce. 



Mantle-lobes separated ; foot large, bent, and deeply grooved. 

 Shell equivalve ; hinge long, with many denticles. 



Genera Area, Cucullcea, Pectunculus, Limopsis, Nucula, Isoarca. 



Family Solenellidce. 



Mantle-lobes united behind, with a single siphonal orifice, or with 

 two short united siphons. Shell equivalve ; hinge edentulous, or with 

 fine sharp teeth. 



Genera Solemya, Solenella, Yoldia, Leda. 



Family Trigoniadce. 



Mantle-lobes separated ; foot long, pointed, sharply bent. Shell 

 equivalve, trigonal ; hinge-teeth few, diverging. 



Genera Trigonia, MyopJioria, Axinus, Lgrodesma. * 



Family Unionidoe. 



Mantle-lobes united between the siphonal orifices and, rarely, in 

 front of the branchial opening ; foot very large, tongue-shaped, com- 

 pressed, byssiferous in the fry. Shell usually equivalve ; anterior 

 hinge-teeth thick and striated, posterior laminar, sometimes wanting. 



Genera Unio, Castalia, Anodon, Iridina, j^theria, Miillerice. 



C. DiMTARiA SiPHONiFERA. With respiratory siphons. 

 a. Siphons short, pallial line simple. 



Family Chamidce. 



Pedal and siphonal orifices small, subequal ; foot very small. Shell 

 inequivalve ; hinge-teeth 2-1, or two in one valve, one in the other. 

 Genera Chama, Monopleura, Diceras. 



Family Hippuritidce. {Rudistes, Lam. ; extinct.) 



Shell inequivalve, unsymmetrical, thick, attached by the right 

 umbo; umbones frequently camerated ; hinge-teeth 1-2. Adductor 

 impressions two, large ; three of the left valve on prominences. 



Genera Hippurites, Radiolites, Caprinella, Caprotina. 



Family TridacnidcB. 



Pedal orifice large ; siphonal orifices surrounded by a thickened 

 pallial border, the anal one with a tubular valve ; foot small^ cylin- 

 drical, byssiferous. Shell equivalve, open; muscular impressions 

 sub-central and blended. 



Genera Tridacna, Hippopus. 



