TRANSACTIONS OF WAGNER 



TERTIARY FAUNA OF FLORIDA 



Subfamily MACTRIN^E. 



Genus MACTBA (L.) Lamarck. 



Typical Mactras, as might be expected from their position in the line of 

 evolution, are relatively modern and are not yet known in America from 

 earlier than Oligocene rocks. Only one small recent species is known from 

 the Atlantic coast, while in the old world the group is numerously represented 

 in the recent fauna. 



Mactra chipolana n. s. 

 PLATE 27, FIGURE 19. 



Oligocene of the Chipola beds, Calhoun County, Florida; Burns. 



Shell rather thin, subovate, compressed, sculptured chiefly by lines of 

 growth which are emphasized at short intervals by being slightly elevated ; 

 judging from recent shells, these lines in life bore fringes of epidermis ; dorsal 

 areas narrow, elongate, the anterior obscurely impressed, the posterior convex 

 with the inner margin depressed, the outer margin marked by a slightly 

 elevated line, parallel to and outside of which at a short distance runs another 

 which extends from the umbo to the posterior ventral margin, much as in 

 Mactrotoma ; interior rather smooth, the pallial sinus wide, rounded, and 

 extending forward nearly to a vertical line from the beaks ; hinge normal, the 

 septum below the ligament inconspicuous, the laterals short and smooth, the 

 left cardinal well developed, prominent ; the accessory lamella thin and usually 

 lost ; the chondrophore not prominent, with slightly raised edges and a small 

 apical roof, over which the ligament was sagittate. Lon. of a well-grown 

 specimen about 45, alt. 35, semidiam. 12 mm. 



Only fragments of the left valve of four individuals were obtained. The 

 measurements are, therefore, only approximate. The shell represents the first 

 step of transition from the Spisula to the Mactra stage, and is, therefore, inter- 

 mediate in its characters between Spisula and Mactrotoma. 



Mactra clathrodon Lea. 

 Mactra clathrodon I. Lea, Contr. to Geol., p. 212, pi. 6, fig. 223, 1833. 



St. Mary's City, Maryland, Finch ; in the Chesapeake Miocene. 



This small species, from an examination of Mr. Lea's types, appears to 

 be a Mactra and not the young of any of the Miocene Spisulas or the S. 

 modicella in particular, to which Conrad referred it. Of the known St. Mary's 

 species it could only be the young of S. pondcrosa Conrad, from which it is 



