442 MOLLUSC A phylum vi 



Meek and Wortlien, and Nemodon Conrad, are allied. Carbonarca Meek and Wortlien. 

 Beaks inflated, curved, angular behind ; hinge margin curved, with two oblique 

 teeth. Carboniferous. 



Cucullaea Lam. (Fig. 660). Shell inflated, trapezoidal ; hinge teeth in the centre 

 of the hinge short, transverse or oblique, the terminal teeth on each side longer, sub- 

 parallel to the hinge line ; posterior adductor usually supported by a radial elevated 

 lamina or buttress. Jura to Recent ; maximum in Mesozoic. 



GucuUaria Desh., of the Eocene, and Idonearca Conrad, are closely allied. 



Family 12. Cyrtodontidae Ulrich. 



Shell equivalve, short, usually heavy, convex and earthy, loithout persistent epidermis, 

 area small, ligament parivincular (?) ; hinge teeth transitional between the Parallelodon 

 and Dysodont type; adductor scars sihb-equal, the posterior larger but less impressed. 

 Ordovician and Silurian. 



These forms are evidently intermediate in character. They recall Liinopsis among later 

 types, are nearly related to the Parallelodontidae, but have not the multivincular ligament ; 

 the hinge has Dysodont elements, but the difference of texture and epidermis stand in the 

 way of assimilating them with Mytilacea. 



Cyrtodonta Bill. (Cypricardites p.p. Conrad ; Palearca Hall). Shell rounded, 

 moderately ventricose, with rather tumid, incurved, anterior beaks ; area narrow and 

 obscure ; cardinal teeth two to four, obliquely curved or horizontal ; lateral teeth near 

 the posterior end of the hinge elongate, strong, curved or oblique ; pallial line simple. 

 Anterior adductor set on the wall of the valve. Ordovician and Silurian. 



Vanuxemia Bill. Beaks more nearly terminal, anterior adductor scar excavated 

 out of the liinge plate. Ordovician. 



TVIiitella and Tschyrodonta Ulrich ; Matheria Billings. Ordovician and Silurian. 



Family 13. Limopsidae Dall. 



Shell pectunculoid, equivalve or nearly so ; the ligament alivincular, partly 

 immersed, its socket approaching a chondrophore ; area small ; foot long, narrow, grooved, 

 byssiferous ; otherwise as in Arcidae. Trias to Recent. 



These forms precede the typical Area and have a special facies of their own. The two 

 dental series of the hinge are often discrepant in character or direction, recalling the 

 Parallelodontidae. 



Limopsis Sasso (Fig. 661). Small, rounded or oval, recalling Glycimeris, except 

 for the alivincular ligament. Trias to Recent. 



Trinacria Mayer {Trigonocoelia Nyst). Like Limopsis, but tri- 

 angular, with the posterior slope keeled. Eocene. Gnisma Mayer, 

 from the Eocene, appears to be related. 



Family 14. Arcidae Dall. 



Shell' trapezoidal or rounded, with the posterior side longer; ligament 



usually multivincular ; hinge typicodly taxodont, with the teeth in two 



Limopsis aurita similar series, meeting below the beaks, and approximately vertical to 



Brocchi. Pliocene; the margin of the valve ; foot stout, short, deeply grooved. Jura to 

 Piacenza. i/.. -r, ^ 



Recent. 



Most of the Paleozoic ^rc«dike forms are probably Parallelodontidae, and the typical 

 Areas are preceded by Pectunculoid forms. The convergence of the types of Arcacea as we 

 recede in geological time is very marked, and their relations to the Nuculacea are evident in 

 spite of the later developed differences. 



Fig. 661. 



