CLASS T 



PELECYPODA 



449 



Posldiniininjd V>v(nn\ {Ahlacomnja Stciiini.) (Fig. 683). EciuivaTve, lliiii, (•(Mii]ir<->('d, 

 coiicentiically wavcd ; liiiigc inargiii sti;iigli1, cdciilulous ; valves not auriculate ; beaks 

 sub-ceiitral, not conspicuous. Sihuian lo Jul■a^i<i(•. Ovcr fifty species are known ; 

 very jjrofiise in tlie Jura-Trias, sometinies forniing massive beds. 



Malleus Lam. ; (?) Pltilohrya Carpenter ; (?) Hochstetteria Velain. Recent. 



heaks 



Aucella mosqucnsis Keys. Upper 

 Jura ; Moscow, Russia. 



Family 22. Myalinidae Frech. 



Shell obliquely ovate, widened hehind, sometinies lüith a small anterior ear 

 anterior or terminal ; hinge edentulous, straight ; area 

 amphidetic, longitudinally grooved ; ligament parivin- 

 cular (?) Adductor scars suh-equal ; hyssal notch distinct. 

 Silurian to Jura. 



Myalina de Kon. Shell thick, ol)lique, with deep 

 adductor scars anteriorly under the terminal beaks. 

 Silurian and Devonian. 



Hoplomytilus Sandb.; Myalinoptera Frech ; Ptycho- 

 desma, Mytilops and Modiella Hall. Devonian. Leio- 

 myalina Frech ; Aphanaia, Posidoniella de Kon. ; Liebea 

 Waagen ; Atomodesma Beyr. Carboniferous. 



Pergamidea Bitt. Thick-shelled, equivalve, inflated ; anterior auricle distinct, 

 sharply truncated ; hinge margin notch ed below the beak. Trias of Asia Minor. 



Mysidia Bitt. Anterior ear reduced. Trias. 



Aucella Keys. (Fig. 684). Tliin, inequivalve, inflated, small, concentrically 

 waved, sometinies with radial Striae. Left valve larger, arcuate, witli very small 

 anterior ear ; riglit 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 ; hyssus 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, 

 irregulär, edentulous, with a triangulär chondrophore for the 

 Eocene to Recent. 



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



s. (? suh Ostreidae.) 



Superfamily 4. OSTRACEA Goldfuss. 



Shell degenerate, sessile, inequivalve, generally edentulous, 



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



,■ 2tt prismatic outer layer ; epidermis inconspicuous ; area amphidetic, 



Miiiich', ligament alivincular ; foot and byssus ahsent ; valves usually 



close-fitting ; mantle lobes free, without siphons. 



ligament 



Fin. 685. 



Vllhrll,, r,,;/hni 



E!,'ypt. -'/o. 



Family 24. Ostreidae Lamank. 

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



l)nvrill,\ //.. Ktudes sur Irs Vulsc'llidc's. Ami. Fiilroiit.. 1907, vol. ii. 



VOL. 1 



2 G 



