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. Decent. 



FIG. 



Aucella Mosquensis, Keys. 

 Jura ; Moscow, Russia. 



Upper 



Family 22. Myalinidae. Freeh. 



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 Devonian. , 



Hoplomytilus, Sandb. ; Myalinoptera, Freeh ; Ptychodesma, Mytilops, Modiella, 

 Hall. Devonian. Leiomyalina, Freeh ; Aphanaia, 

 Posidoniella, de Kon. ; Liebea, Waagen ; Atomodesmc^ 

 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, inequi valve, 

 inflated, small, concentrically waved, sometimes with 

 radial 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, inequi- 

 valve, with an alivincular ligament ; byssus wanting ; otherwise 

 as in the Pteriidae. Tertiary to Kecent. 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 Rainc. Eocene. ? Chalmasia, Stol. Cretace- 

 ous. (? Ostreidae.) 



Superfamily 4. OSTRACEA. Goldfuss. 



Shell degenerate, sessile, inequivalve, generally edentulous, 

 wings obsolete; with a sub-nacreous or porjellanous inner and 

 prismatic outer layer ; epidermis inconspicuous ; area amphidetic, 

 ligament alivincular; foot and byssus absent; valves usually 

 close-fitting ; mantle lobes free, without siphons. 



FIG. 



Vulsella Caillaudi, Zitt. 

 Lower Eocene ; Minich, 

 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. 



