364 



MOLLUSCA 



SUB-KINGDOM VI 



Yoldia, Holler (Fig. 608). Shell thin, wide, and more or less gaping behind, 

 hinge as in Nucula. Cretaceous to Recent. 



FIG. 606. 



Leda rostrata, Lam. 

 sp. Middle Jura ; 

 Milhaucl, Aveyron. 



Vi- 



FIG. 607. 



Leda Deshayeslana, Duch. 

 Oligocene ; Rupelmonde, 

 Belgium, i/j. 



FIG. 608. 



Yoldia arctica, Gray. 

 Pleistocene ; Bonus- 

 lan, Sweden, i/x. 



FIG. 609. 

 Nuculina ovalis, 

 Wood sp. Mio- 

 cene; Forchtenau, 

 near Vienna. 



Nuculina, d'Orb. (Fig. 609), Nuculiform, hinge teeth few and. discrepant ; 

 with large lateral tooth and external ligament. Tertiary and Recent. 



Malletia, Desm., and Tindaria, Bell. Yoldiform and Nuculiform respectively, but 

 without internal chondrophore. Tertiary and Recent. 



Superfamily 2. ARCACEA. Deshayes. 



Shell of varied form, usually with a pilose epidermis, porcellanous, with tubuliferous 

 non-prismatic external layer ; area typically amphidetic, ligament external, ali- or multi- 

 vincular ; gills fllibranchiate, with the filaments usually reflected ; mantle lobes free, without 

 siphons, the pallial line simple; foot variable, deeply grooved, byssogenous ; marine or 

 fiuviatile. 



Family 11. Parallelodontidae. Dall. 



Shell Arciform, with the posterior hinge teeth elongated, tending to be parallel to the 

 margin ; ligament multivincular. Carboniferous to Recent. 



FIG. 610. 



ParaUelodon Hirsonensis, Morris and 

 Lye. Great Oolite ; Minchinhampton, 

 England. 1/1- 



The ancient forms of this group appear to connect 

 with the Pteriacea through Pterinea, and with Area 

 through Cucullaea. The recent forms, which from 

 their shell characters have been referred to Macrodon, 

 are all small and abyssal. The relationship of this 

 family to the Arcidae is very intimate but not 

 exclusive. 



ParaUelodon, Meek (Macrodon, Lycett), (Fig. 

 610). Shell elongate, sub-quadrate, with amphi- 

 detic area, and prominent, rather anterior beaks. 

 Anterior teeth transverse or flexuous, posterior long and parallel to the hinge line. 

 Devonian to Tertiary;, maximum in Coal Measures. 







