XVI PSEUDOLAMELLIBRANCHIATA 449 



branchiae with interfoliary junctions, genital glands penetrating 

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

 Fam. Mytilidae. — Byssus well developed, shell more or less 



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



Principal genera : 3Tytilus^ ? Myalina^ Septifer^ Modiola^ Litliodo- 

 mus, Orenella^ Dacrydium^ 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 kidnej-s. 



Fam. 1. AvicuUdae. — 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. 2>(i2. — Avicxda lieteroptera Lam., Aiis- 



Avicula, including Melea- ':^^:^^ '^'^'^^ ^ '^^ 

 grina., Malleus ; Vulsella (no 



wings or hinge-teeth),; Perna^ including Crenatula^ Inoceramus 

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

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

 zoic); Pinna; Aviculopinna (Carboniferous). 



Fam. 2. Prasmidae. — 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 



