442 MOLLUSCA phylüm vi 



Meek and Worthen, and Nemodon Conrad, are allied. Garhonarca Meek and Wortlien. 

 Beaks inflated, curved, angiüar beliind ; hinge margin curved, witli two oblique 

 teetli. Carboniferous, 



Gucullaea Lani. (Fig. 660). Sliell inflated, trapezoidal ; liinge teetli in tlie centre 

 of tlie 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. 



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



Family 12. Oyrtodontidae 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 suh-equal, the posterior larger hut less impressed. 

 Ordovician and Silurian. 



These forms are evidentlj" 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 ditference of texture and epiderniis stand in the 

 way of assimilating them with Mytilacea. 



Gyrtodonta Bill. (Gypricardites 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 teetli near 

 the posterior eiid 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 hinge plate. Ordovician. 



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



Family 13. Limopsidae Dali. 



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

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

 byssiferous ; otherwise as in Ärcidae. 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- 

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

 from the Eocene, appears to be related. 



Family 14. Arcidae Dali. 



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



Fig. 661. usually multivincular ; hinge typically taxodont, with the teeth in two 



Limopsis aurita ^'^'^'^^o^i' series, meeting helow the heaks, and approximately vertical to 



pL^Sza^^iT^"^' ^^^ inargin of the valve; foot stout, short, deeply grooved. Jura to 



Recent. 



Most of the Paleozoic ^rca-like forms are probably Parallelodontidae, and the typical 

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

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

 spite of the later developed ditferences. 



