THE LAMELLIBRANCHIA 



271 



FAMILY 1. CARDIIDAE, Gray, The mantle slightly closed ; siphons 

 very short and surrounded by a single circle of papillae which are often 

 oculiferous (Fig. 243, o.t}. Foot very long, geniculated. Pallial line of 

 the shell without a sinus ; two adductor muscles. Genera Cardium, 

 Linnaeus ; adductor muscles subequal ; British (Fig. 243). Psevdokellya, 

 Pelseneer (Fig. 221). Both Byssocardium, Munier-Chalmas, and Litho- 

 cardium, Woodward, fossils from the Eocene, have a much reduced anterior 

 adductor muscle. FAMILY 2. LIMNOCARDIIDAE, Stoliczka. Siphons very 

 long, united throughout their extent. Shell gaping ; two adductor 

 muscles. Inhabitants of brackish waters. Genera Adacna, Eichwald ; 

 from the Caspian Sea. Limnocardium, Stoliczka ; from the Caspian Sea 

 and fossil from the Tertiary. Arcicardium, Fischer ; fossil from the 

 Tertiary. FAMILY 3. TRIDACNIDAE, Broderip. Mantle closed to a con- 

 siderable extent, the orifices distant from one another ; no siphons. The 

 foot short, with a more or less well developed byssus. A single adductor 



Fio. 243. 



Cardium etlule, left-side view. ..<?, anal siphon ; br.s, branchial siphon ; ;, foot ; li, ligamen 

 of the shell ; o.t, eye-spots on the tentacles ; .sfc, shell. (Alter Deshayes.) 



muscle. The gills narrow. The shell thick. Genera Tridacna, 

 Bruguiere ; byssus stout ; shell gaping anteriorly ; from the Indian 

 and Pacific Oceans. Hippopus, Lamarck ; byssus reduced ; shell not 

 gaping. 



SUB-ORDER 6. CHAMACEA. 



Asymmetrical, inequivalve, fixed Eulamellibranchia, with extensive 

 pallial sutures and distant pallial orifices ; no siphons. Two adductor 

 muscles present. The foot reduced and without a byssus. Shell thick, 

 without a pallial sinus. 



FAMILY 1. CH AMID AE, Gray. Shell with subequal valves and promi- 

 nent umbones more or less spirally coiled ; ligament external. Genera 

 Chama, Bruguiere ; the free valve only slightly ventricose ; shell 

 lamellated or spiny. JHceras, Lamarck; shell smooth, the umbones 

 largely divergent and coiled ; the adductor muscles (at least the anterior) 

 attached to myophorous apophyses ; fossil from the Jurassic (Fig. 244, A). 

 Requienia, Matheron ; the fixed valve spirally coiled ; the free valve 



