ORDER I PRIONODESMACEA s 365 
Granumatodon, Meek and Worthen, and 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 Idonearea, 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. 
Whatella, 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, 
grooved, byssiferous ; otherwise as in Arcidae, Trias to Recent. 
These forms precede the typical Arca 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- Fic. 612. 
angular, with the posterior slope keeled. Eocene. Cnisma, Mayer, — Limopsis aurita, 
from the Eocene, appears to be related. Be Scene, 
Piacenza. 1/;. 
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 Arca-like forms are probably Parallelodontidae, and the typical 
Arcas 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 Nwculacea are evident in 
spite of the later developed differences. 
