THE LAMELLIBRANCHIA 



277 



tube secreted by the siphons ; pallial line sinuous. Genera Clavagella, 

 Lamarck ; left valve fused to the tube ; adductor muscles well developed ; 

 a boring form. Brechites, Guettard ( = Aspergillum, Lamarck) ; the two 

 valves fused to the tube and external (Fig. 248) ; no posterior adductor 

 muscle ; the anterior adductor much reduced ; the anterior extremity 

 bearing numerous tubular projections serving for adhesion ; Indian and 

 Pacific Oceans. FAMILY 12. LYONSIIDAE, Fischer. Mantle largely 

 closed, with a fourth pallial orifice ; siphons short, invertible ; foot 

 byssiferous. Shell thin, granular externally ; the pallial sinus feeble ; 

 the ligament internal. Genera Lyonsia, Turton ; shell regular and 

 elongated; British (Fig. 219). Entodesma, Philippi ; shell irregular, 

 truncated behind ; a boring form, sometimes found in the tests of 

 Ascidians. Mytilimeria, Conrad ; shell regular, ventricose, gaping behind. 

 FAMILY 13. VERTICORDIIDAE, Wood. Siphons short ; the gills papillose ; 



CLCL 



pet, 



FIG. 249. 



Poromya tornata, left-side view, a.a, anterior adductor; a.p, anterior labial palp; a.s, 

 anal siphon ;/, foot; g.l, gill lamellae on the septum; ft^ heart; ha, posterior adductor; in, 

 intestine; li, liver; pa, pallial suture; p.p, posterior labial palp; p.t, pallial tentacles; r.p, 

 retractor posterior pedis ; r.s, retractor of the septum ; s, septum ; vj, valvular fold of the 

 branchial aperture. 



foot small ; palps well developed. Shell globular, very slightly gaping, 

 without a pallial sinus. Many species abyssal. Genera Verticordia, 

 Wood ; mantle largely closed ; the pedal orifice small. Euciroa, Dall ; 

 heart situated above the rectum. Lyonsiella, Sara ; foot byssiferous. 

 Halicardia, Dall. 



ORDER 4. Septibranchia. 



The Septibranchia are dimyarian Lamellibranchs in which the 

 mantle remains fairly open and has two sutures and two siphons. 

 The foot is long and slender ; the byssus rudimentary or absent. 

 The pallial line is simple or very slightly sinuous. The essential 

 character of the group is the disappearance of the gills as respiratory 

 organs, a character which is not found in any other Lamellibranch. 

 The gills are transformed into a muscular septum (Fig. 249, s) 



