372 MOLLUSCA SUB-KINGDOM VI 
concentrically waved ; hinge margin straight, edentulous ; valves not auriculate ; beaks 
sub-central, not conspicuous. Silurian to Jurassic. Over fifty species are known ; very 
profuse in the Jura-Trias, sometimes forming massive beds. 
Malleus, Lam.; Philobrya, Carpenter ; Hochstetteria, Velain. Recent. 
Family 22. Myalinidae. Frech. 
Shell obliquely ovate, widened behind, sometimes with a small anterior ear; beaks 
anterior or terminal; hinge edentulous, straight; area amphidetic, longitudinally 
grooved ; ligament parivincular?  Adductor scars sub-equal; byssal notch distinct. 
Silurian to Jura. 
Myalina, de Kon. Shell thick, oblique, with deep adductor scars anteriorly under 
the terminal beaks. Silurian and Deyonian.° 
Hoplomytilus, Sandb.; Myalinoptera, Frech; Ptychodesma, Mytilops, Modiella, 
Hall. Devonian. Leiomyalina, Frech; Aphanaia, 
Posidoniella, de Kon.; Liebea, Waagen; Atomodesma, 
Beyr.; Anthracoptera, Salter. Carboniferous. 
Pergamidea, Bitt. Thick-shelled, equivalve, inflated ; 
anterior auricle distinct, sharply truncated ; hinge margin 
notched below the beak. Trias of Asia Minor. 
Mysidia, Bitt. Anterior ear reduced. Trias. 
Aucella, Keys. (Fig. 635). Thin, inequivalve, 
SNEED inflated, small, concentrically waved, sometimes with 
Jace iMcssow, Hasta, ae Upper yadial striae. Left valve larger, arcuate, with very small 
anterior ear; right valve flatter and smaller. Area 
short, striated, with a ligamental sulcus below the beak. Upper 
Jura and Cretaceous ; distribution world wide. 

Family 23. Vulsellidae. Adams. 
Shell Ostreiform, not alate, monomyarian, edentulous, inequt- 
valve, with an alivincular ligament ; byssus wanting ; otherwise 
as in the Pteriidae. Tertiary to Recent. A degraded type 
which has become specialised through commensalism with 
Sponges. 
Vulsella, Lam. (Fig. 636). Shell vertically produced, 
irregular, edentulous, with a triangular chondrophore for the 
ligament. Eocene to Recent. 
Vulsellina, de Raine. Eocene. ? Chalmasia, Stol. Cretace- 
ous. (2 Ostreidae.) 
Superfamily 4. OSTRACEA. Goldfuss. 
Shell degenerate, sessile, inequivalve, generally edentulous, 
wings obsolete; with a sub-nacreous or porcellanous inner and 
prismatic outer layer ; epidermis inconspicuous ; area amphidetic, Fic. 636. 
ligament alivincular ; foot and byssus absent; valves usually _ Vulsella Coillaudi, Zitt. 
eas : . Lower Eocene; Minich, 
close-fitting ; mantle lobes free, without siphons. Egypt. 2/3 

Family 24. Ostreidae. Lamarck. 
Shell distorted by early adherence to other objects ; monomyarian, the anterior adductor 
absent ; edentulous, or with obscure Schizodont dentition ; dimyarian when young ; the 
foot obsolete or absent in the adult. Carboniferous to Recent. 
