XVI PSEUDOLAMELLIBRANCHIATA 449 



branchiae with interfoliaiy junctions, genital glands penetrating 

 the side of the mantle and opening by the side of the kidneys. 

 Ya^i. Mytilidae. — Byssus well developed, shell more or less 



equivalve, oval, broad ; hinge-teeth evanescent. Devonian . 



Principal genera: MytilusJ Myalina, Septifer, Modiola, Lithodomus, 

 Crenella, Dacrydliun, Myrina, Idas, Modiolaria, Modiolarca. 



Order III. Pseudolamellibranchiata 



Mantle edges entirely open, foot little developed, anterior 

 adductor usually aborted, branchial filaments reflected, with inter- 

 lamellar junctions, which are sometimes vascular ; genital glands 

 opening into the kidneys or close to the apertures of the kidneys. 



Fam. 1. Avicididac. — Foot long, tongue-shaped, byssogenous 

 apparatus well developed, branchiae concrescent with the mantle, 

 adductor muscle sub-central, at times a small anterior adductor, 

 siphons absent ; shell usually 

 inequivalve, dorsal margin r~r 

 straight, often very long, ~'"''^^ 

 winged, lateral teeth much 

 prolonged ; structure of shell 

 cellular, inside prismatic, out- 

 side nacreous. Palaeozoic 



Principal o-enera : ^^^* ^^2. — Avicula heteroptera Liim., Australia, 



. . ^ Tir 1 showing the inequivalve shell and byssal 



Avicula, including Melea- gjuus (j.s). 

 grina, Malleus; Vid sella (no 



wings or hinge-teeth) ; Ferna, including Crenahda, Inoceramus 

 (ligaments in a number of fossettes) ; Aucella and Monotis 

 (Palaeozoic and Secondary) ; Pterinaea and Amhonychia (Palaeo- 

 zoic) ; Pinna ; Avic^dojnnna (Carboniferous). 



Pam. 2. Prasinidae. — Shell very small, umbones anterior, in- 

 curved, anterior side depressed, hinge-teeth replaced by dentiform 

 projections of the lunule fitting into corresponding grooves. 

 Eecent. Single genus, Prasina. 



Fam. 3. Ostreidae. — Heart generally ventral to the rectum, 

 branchiae concrescent with the mantle, no byssus ; shell ine(pti- 



valve, fixed by the left valve, form irregular. Jurassic- . 



Genera : Ostrca ; Hcllgmus (Oolite), Naiadina (Cretaceous), Pern- 

 ostrea (Jurassic). 



FA:\r. 4. Pectinidac. — Byssus usually absent, mantle edge open, 



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