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.  Srechites, Guettard (= Aspergillwm, 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. Faminy 12. lLyonsimpar, 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). Hntodesma, 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 ; 
Fic. 249, 
Poromya tornata, left-side view. a.a, anterior adductor; a.p, anterior labial palp; a.s, 
anal siphon ; f, foot; g./, gill lamellae on the septum; h, 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; v./, 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. Huciroa, Dall ; 
heart situated above the rectum. Lyonsiella, Sars; 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) 
