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860 



TERTIARY FAUNA OF FLORIDA 



arms of the cardinal seem (when different sections of the genus are compared) 

 to rotate, as it were, on an imaginary axis nearly coincident with the angle 

 at which the two teeth are soldered together. From this it results that the 

 anterior arm of the consolidated cardinal may be coalescent with the dorsal 

 shell margin (in which case the anterior lamina: will radiate from the ventral 

 sinus), or may be superimposed exactly upon one of the -lamina; (usually the 

 ventral one, when the hinge appears to have lost one of its anterior laminae), 

 or may rise from the hinge-plate on a radial line behind that of the ventral 

 anterior lamina. Concurrently the posterior arm of the tooth may be directed 

 vertically downward with an empty triangular space between it and the ante- 

 rior verge of the pit (in which case there is often an accessory lamella at the 

 verge), or it may rise directly from the verge like a wall, or it may project out 

 over the pit supported only by its attachment to the cardinal spur. The 

 angle at which the two arms of the cardinal tooth unite is usually quite con- 

 stant in the same species, and the triangle or ventral sinus enclosed between 

 them is therefore quite uniform in shape. 



By following this rather long but necessary dissertation upon the ex- 

 amples of the hinge, which have been figured and lettered for the pur- 

 pose, it will be found comparatively easy to apply the terms used to the 

 description of particular hinges and to comprehend the relations of the several 

 parts. 



The earliest Mactrida yet recognized are Mesozoic. In the Chico beds 

 of California, now thought to represent the lower part of the Upper Creta- 

 ceous, are several genuine Mactras, described by Gabb under the name of 

 Cymbophora. In the early Eocene Ptcropsis Conrad hardly differs from Raeta. 

 In the Middle Tertiary a very large number .of species are said to occur, ex- 

 ceeding the recent forms in abundance; many of these, however, are probably 

 synonymes and should not be counted. The group is an essentially modern 

 one, and is probably represented to-day by as many living species as were 

 present in any antecedent fauna. 



A number of groups which should properly belong to the family have 

 been scattered in different families, or even orders, by systematists devoted to 

 single characters or morphologists having little acquaintance with the details 

 of character. A number of groups which have closely related shells present 

 marked differences of superficial anatomy, as, for instance, the Mesodcsinat'nln-, 

 which have Mactroid shells and Tellinoid free siphons; others defy final 

 classification, owing to our ignorance of their anatomy. The place of various 



