CLASS I 



PELECYPODA 



449 



Fosidonomya Bronn {Ahlacomya Steiiim.) (Fig. 683). Eqiiivalve, tliin, compressed, 

 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 Freeh. 



Shell obliquely ovate, widened behind, sometimes with a small anterior ear ; bealcs 

 anterior or terminal ; hinge edentulous, straight ; area 

 amfhidctic, longitudinally grooved; ligament parivin- ^^fi^^^^'^TJ^ 

 cular (?) Adductor scars sub-eqiud ; byssal notch distinct. 

 Silurian to Jura. 



Fig. 684. 



Aucella mosrjuensis Keys. Upper 

 ■Jura; Moscow, Russia. 



; anterior auricle distinct, 

 Trias of Asia Minor. 



Aucelkt Keys, 

 waved, sometimes 



Myalina de Kon. Shell thick, oljlique, with deep 

 adductor scars anteriorly under the terminal beaks. 

 Silurian and Devonian. 



Hoplomytilus Sandb.; Myalinoptera Freeh ; Ptycho- 

 desma, Mytilops and Modiella Hall. Devonian. Leio- 

 myalina Freeh ; Aplianaia, Posidoniella de Kon. ; Liebea 

 Waagen ; Atomodesma Beyr. Carboniferous. 



Pergamidea Bitt. Thick-shelled, equivalve, inflated 

 sharply truncated ; hinge margin notched below the beak. 

 Mysidia Bitt. Anterior ear reduced. Trias. 



(Fig. 684). Thin, inequivalve, inflated, small, concentrically 



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, tvith 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. 685). Shell vertically produced, 

 irregular, edentulous, with a triangular chondrophore for the 

 ligament. Eocene to Recent. 



Vulsellina de Rainc. Eocene. (?) Ghalmasia Stol. Creta- 

 ceous. (? sxhb Ostreidae.) 



Superfamily 4. OSTRACEA Goldfuss. 



Shell degenerate, sessile, inequivalve, generally edentulous, 

 Pjp^ ggg loings obsolete ; tvith a sub-nacreous or porcellanous inner and 



Vulsella caillaudi zitt prismatic outer layer ; epidermis inconspicuous ; area amphidetic, 

 Lower Eocene ; Miiiich, ligament alivinculav ; foot and byssus absent ; valves usually 



close-fitting ; mantle lobes free, without siphons. 



' Family 24. Ostreidae Lamarck. 

 Shell distorted by ea,rly adherence to other objects ; monomyarian, the anterior 

 ^ Douvllle, II.., Etudes sur les Vulsellidt'S. Ann. Pak'out., 1907, vol. ii. 



VOL. I 



2 G 



