INSTITUTE OF SCIENCE, PHILADELPHIA. 517 



Carboniferous to recent fauna. 

 Ex. Macrodon, Parallelodon, Oinalia, Nemodon, Cucullaria, Cucul/fea, etc. 



The ancient forms of this group appear to connect with the Pteriacea 

 through Pterinea, to Area through Cuculfcsa. The recent forms, which from 

 their shell characters have been referred to Macrodon, are all small and abyssal. 

 Their soft parts agree with Area and the animal is byssiferous. The relation- 

 ship of this family to the Arcidce is very intimate but not exclusive. 



FAMILY CYRTODONTID^. 



Shell equivalve, short, usually heavy, convex and earthy, without persis- 

 tent epidermis, area small, ligament parivincular ? ; hinge-teeth transitional 

 between the Macrodon and Dysodont type ; adductor scars subequal, the pos- 

 terior larger but less impressed. 



Lower Silurian and Devonian. 

 Ex. Cyrtodonla, Ischyrodonta, Vanuxemia, Whitella. 



These forms are evidently intermediate in character. They recall Limop- 

 sis among later types, are nearly related to Macrodontidce, 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 to the Mytilacea. 



FAMILY LIMOPSID^. 



Shell pectunculoid, equivalve, or nearly so ; the ligament alivincular, part- 

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

 produced at the anterior and posterior ends, arcuate, narrow, grooved, byssifer- 

 ous; palpi reduced ; ventricle embracing the rectum, otherwise as in Arcidce. 



Trias to recent fauna. 



Ex. Limopsis, Trigonocazlia, fCnisma, 



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 Macrodontidce . 



FAMILY ARCID/E. 



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

 usually multivincular ; hinge typically taxodont, with the teeth in two similar 

 series, meeting below the beaks, and approximately vertical to the margin of 

 the valve; ventricle more or less double and dorsal to the rectum ; foot stout, 

 short, deeply grooved. 



Mesozoic ?, to recent fauna. 

 Ex. Area, Barbatia, Scaphula, Senilia, Argina, Attadara. Pectunculus, etc. 



Most of the Paleozoic forms are probably Macrodontidce, the Pectunculoid 

 forms precede the typical Areas. The convergence of the types of Arcacea as 



