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TERTIARY FAUNA OF FLORIDA ^7 



part of their length ; the margin of the mantle largely free, more or less papil- 

 lose; the foot large, hatchet-shaped, not byssiferous. The nymphs and adjacent 

 teeth are sometimes corrugated and the posterior right and anterior left dorsal 

 margins of the valves beyond the hinge-plate are often grooved to receive the 

 bevelled edge of the valve opposite. The shells are always porcellanous. Many 

 names have been given to the different mutations of the type, of which some 

 among the most familiar, as Dione, Callista, Caryatis, and Cytherea, are pre- 

 occupied in other cases. 



The forms which are precursors occur as early as the Cretaceous, and even 

 possibly in the Jura, but most of these early forms are not typical, and the 

 genera really begin to assume typical form only in the Eocene. 



It is known from the researches of Bernard that the anterior and posterior 

 teeth of the same valve are originally continuous laminae; thus the superior 

 lamina of the left valve breaks up into the posterior cardinal and the anterior 

 lateral, while the inferior lamina divides to form the two other cardinals of 

 that valve. Ordinarily, the primary connections are lost sight of in the adult, 

 and the cardinal teeth appear to spring from an imaginary centre under the 

 hinge-margin above them. In certain groups, such as Callocardia, Atopodonta, 

 or V 'eneriglossa, however, the anterior and posterior right cardinals remain 

 connected as well as the anterior and middle left cardinals, and when the valves 

 are closed the former are inserted above the latter and between them and the 

 hinge-margin, while the middle right cardinal fits in below the united pair of 

 the left valve, thus giving an odd look to the hinge, the reason for which 

 requires some study to recognize. As a whole the Meretrix group represents 

 an earlier type than typical Venus and one with somewhat more archaic hinge 

 characters. Of these Callocardia is unquestionably the least developed. 



Owing to the weight of other characters and the fact that no linear arrange- 

 ment can adequately represent the intricate relationship of such a group as the 

 yeneridce, I have not placed Callocardia at the head of the subfamily, but rather 

 at the head of the portion of the series following Meretrix, to which it seems, 

 by other characters, to be allied. It may be well, however, to contrast the hinge 

 characters that their features may be clearly understood. 



The subfamily as a whole is distinguished from the Veneridcs by the in- 

 variable presence of one or more anterior lateral teeth. - This tooth, when there 

 is but one, is on the hinge-plate of the left valve and is received into a pit, or 

 between two much more feeble anterior laterals in the right valve. In such 

 groups as Cytherea Bolten the anterior lateral is degenerate, and in senile 

 specimens nearly obsolete, but traces of it may always be detected in youthful 



