THE LAMELLIBRANCHIA 27 
Faminy 1. Carprpar, Gray. The mantle slightly closed ; siphons 
very short and surrounded by a single circle of papillae which are often 
oculiferous (Fig. 243, 0.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). Psewdokellya, 
Pelseneer (Fig. 221). Both Byssocardiwm, Munier-Chalmas, and Litho- 
cardium, Woodward, fossils from the Eocene, have a much reduced anterior 
adductor muscle. Famriy 2. LimnocarpiDA®, 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. Faminy 3. Trrpacnipak, 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 
Fie. 243, 
Cardium edule, left-side view. «.s, anal siphon ; br.s, branchial siphon ; f, foot ; i, ligamen 
of the shell ; 0.t, eye-spots on the tentacles; sh, shell. (After Deshayes.) 
muscle. The gills narrow. The shell thick. Genera — Tridacna, 
Bruguiére ; byssus stout; shell gaping anteriorly; from the Indian 
and Pacific Oceans. Hippopus, Lamarck; byssus reduced; shell not 
gaping. 
Susp-OrRDER 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. 
Famity 1. Caamipar, 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. Jiceras, 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). 
Requienta, Matheron ; the fixed valve spirally coiled; the free valve 
