XVI PSEUDOLAMELLIBRANCHIATA 449 



branchiae with interfoliary junctions, genital glands penetrating 

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

 Fam. 3Iytilidae. — Byssus well developed, shell more or less 



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



Principal genera : Mytilus^ ? Myalina^ Septifer^ Modiola^ Lithodo- 

 mus, Orenella, Dacrydium, Myrina, Idas, Modiolaria, Modiolarca, 



Order III. Pseudolamellibrancliiata 



Mantle edges entirely open, foot little developed, anterior 

 adductor usually aborted, branchial filaments reflected, with inter- 

 lameliar junctions, which are sometimes vascular ; genital glands 

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



Fam. 1. Aviculidae. — 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 

 in equivalve, dorsal margin 

 straight, often very long, 

 winged, lateral teeth much 

 prolonged ; structure of shell 

 cellular, inside prismatic, out- 

 side nacreous. Palaeozoic 



. Principal genera: Fig. Z(i2. — Avicula heteroptera Lam., Aus- 



Avicula, including Melea- ^yS^S)!''^ '"'''"™'™ ^'^'' ''"' 

 grind, Malleus ; Vidsella (no 



wings or hinge-teeth) ; Perna, including Crenatula, Inoeeramus 

 (ligaments in a number of fosse ttes) ; Aucella and Monotis 

 (Palaeozoic and Secondary); Pterinaea Siud Amb onychia (Palaeo- 

 zoic); Pinna; Aviculopinna (Carboniferous). 



Fam. 2. Prasinidae. — Shell very small, umbones anterior, 

 incurved, anterior side depressed, hinge-teeth replaced by denti- 

 form projections of the lunule fitting into corresponding grooves. 

 Recent. Single genus, Prasina. 



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

 branchiae concrescent with the mantle, no byssus ; shell inequi- 



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



Genera: Ostrea ; Heligmus (Oolite), Naiadina (Cretaceous), 

 Pernostrea (Jurassic). 



Fam. 4. Pectinidae. — Byssus usually absent, mantle edge 



VOL. Ill 2G 



