274 



THE LAMELLIBRANCHIA 



closed ; siphons long and united throughout their length ; a fourth 

 opisthopodial pallial aperture. Foot rather large, compressed. Shell 

 elongated, with a deep pallial sinus and a spoon-shaped projection for the 

 ligament on each valve. Genera Lutraria, Lamarck ; British. Tresus, 

 Gray. Standella, Gray. FAMILY 5. SOLENIDAE, Leach. Elongated, 

 burrowing animals. The foot more or less cylindrical and powerful, 

 without a byssus. Gills narrow. Shell long, truncated, and gaping at 

 each end ; the ligament external. Genera Solenocurtus, de Blainville ; 

 siphons large, partially united, incompletely retractile ; pallial sinus deep ; 

 foot very large and linguiform (Fig. 194); British. Tagelus, Gray; 

 posterior extremity short ; pallial sinus very deep ; estuarine. Ceratisolen, 

 Forbes and Hanley ; siphons long, separate ; gills rather short and not 

 folded ; British. Cultellus, Schumacher ; siphons rather short ; extremity 

 of foot dilated ; British. Siliqua, Megerle ; siphons of medium length ; 

 foot dilated ; shell compressed. Solen, Linnaeus ; siphons short ; foot 

 elongated ; shell rectilinear, cylindrical ; the umbones anterior and 

 terminal ; British. Ensis, Schumacher ; siphons very 

 short ; a fourth pallial orifice ; shell arcuate, the um- 

 bones anterior and sub-terminal ; British. FAMILY 6. 

 SAXICAVIDAE, Gray. Mantle extensively closed ; with 

 a small pedal orifice (Fig. 246, /); siphons elongate, 

 covered by a chitinous sheath and wholly or largely 

 united ; gills prolonged into the branchial siphon. 

 Foot small. Shell gaping, with an external ligament. 

 Genera Saxicava, Fleuriau ; bores holes in rocks ; 

 siphons free at their extremities ; foot byssiferous ; 

 British (Fig. 246). Glycimeris, Lamarck ; siphons 

 very long and completely united ; a burrowing form. 

 Cyrtodaria, Daudin ; shell inequilateral ; the anterior 

 side the longer ; siphons united, incompletely retractile. 

 FAMILY 7. GASTROCHAENIDAE, Gray. Shell thin, 

 Saxicava arctica, ven- without teeth, gaping widely at the anterior end. 

 sTphon^'br's branchial -A-^erior adductor much reduced. Foot small and 

 siphon; by, byssal without a byssus. Gills narrow. Mantle extensively 

 mantle ; sh, shell.' pa ' c l se( i i with long united siphons. Genera Gastro- 

 chaena, Spengler ; a boring form, with a cylindrical 

 foot ; the shell regular and rarely enclosed in an adventitious tube ; 

 British. Fistulana, Bruguiere ; a burrowing form with a very small 

 compressed foot ; shell with a denticulate border, always enclosed in a 

 regular, non-adherent, fragile, club-shaped adventitious tube. 



FIG. 246. 



SUB-ORDER 8. ADESMACEA. 



Eulamellibranchia with very long united siphons and a largely 

 closed mantle. The foot short, truncated, discoid, and without a 

 byssus. The gills prolonged into the branchial siphon. The shell gaping 

 and devoid of a ligament, but with a styloid apophysis in the umbonal 

 cavities. 



FAMILY 1. PHOLADIDAE, Adams. Shell capable of containing all 



