ORDER i PRIONODESMACEA 365 



Grammatodon, Meek and Worthen, arid Nemodon, Conrad, are allied. Carbonarca, 

 Meek and Worthen. Beaks inflated, curved, angular behind ; hinge margin curved, 

 with two oblique teeth. Carboniferous. 



Cucullaea, Lam. (Fig. 611). 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. 



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



Family 12. Cyrtodontidae. Ulrich. 



Shell equivalve, short, usually heavy, convex, and earthy, without persistent epidermis, 

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

 Dysodont type ; adductor scars sub-equal, the posterior larger but less impressed. Ordovician 

 to Devonian. 



These forms are evidently intermediate in character. They recall Limopsis 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 the Mytilacea. 



Cyrtodonta, Bill. (Cypricardites, p.p. Conrad ; Megalomus, 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. Silurian. 



Cypricardites, Conr. (Palearca, Hall). Five cardinal teeth present, the anterior 

 largest. Silurian. 



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

 out of the hinge plate. Ordovician. 



Whitella, Ischyrodonta, Ulrich ; and Matheria, Bill. 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, 

 yrooved, 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, Sassi (Fig. 612). Small, rounded, or oval, recalling 

 Glycimeris, except for the alivincular ligament. Trias to Recent. 



Trinacria, Mayer (Trigonocoelia, Nyst). Like Limopsis, but tri- FIG. 612. 

 angular, with the posterior slope keeled. Eocene. Cnisma, Mayer, Livwpsis aurita, 

 from the Eocene, appears to be related. 



Family 14. Arcidae. Dall. 



Shell trapezoidal or rounded, with the posterior side longer ; ligament usually multi- 

 vincular ; hinge typically Taxodont, with the teeth in two similar series, meeting below the 

 beaks, and approximately vertical to the margin of the valve ; foot stout, short, deeply 

 grooved. Jura to Recent. 



Most of the Palaeozoic ^4rca-like 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. 



