TRANSACTIONS OF WAGNER 

 896 



TERTIARY FAUNA OF FLORIDA 



This group being anterior on the line of evolution to the typical Mactroids 

 is much better represented in the fossil state, and, in the form of its subgenus, 

 Cymbophora Gabb, recedes to the middle of the Cretaceous at least. With 

 these older forms we are not at present concerned, but it may be useful to 

 refer to a group of small Eocene species which were among the first to be 

 described of American fossil species. Omitting ]\Iactra icqnorca and nrtiliiic- 

 tiris, which seem to belong in the Mi-sutlt-siiKitida;, the following species were 

 described at an early date from the Claibornian. M. parilis Conr. (+ [>vgitza 

 Lea), M. decisa, and prcetauiis Conr., all of which belong to the genus Spix/i/a, 

 and even to its typical section, as I have determined by an examination of the 

 type specimens. In other Eocene beds are Sfisula albit upiana Harris from 

 White Bluff, Arkansas ; Spisuln mississippiensis Conr. and S. fitncrata Conr. 

 from Vicksburg, Mississippi, a variety of the latter having been named i/i- 

 cquilatcralis by O. Meyer. There is another form which differs barely, if at 

 all, from funcrata, in the Jacksonian. M. dcntata Lea was founded on the 

 hinge-plate of Pteropsis pafyria Conr. of the Claibornian. 



In the Miocene we find a more numerous and richer development of the 

 genus. 



Subgenus HEMIMACTRA Swainson. 



This comprises a group of species which differ from the typical Sfisula in 

 being thinner, usually larger and more elongated, and agree with it in having 

 the lateral lamina; cross-striated, while in the section Mactromeris Conrad 

 they are smooth, though this character is not one to which I attach any great 

 importance. 



The following species have smooth laminae, and the large recent 5. walls 

 Gould also retains this character. 



Section Miu-liviiicj-h Connul. 

 Spisula (Hemirnactra) dodona n. s. 



PLATE 27, FIGURES 7, 13, 25. 



Oligoccne sands of Oak Grove, Santa Rosa County, Florida; Burns. 

 Shell of moderate size, compressed, subtriangular, arcuate, nearly smooth 

 or with fine incremental lines, subequilateral; the anterior side a trifle shorter, 

 anterior slope impressed, slightly concave, anterior end rounded; base arcuate ; 

 posterior slope convex, mcsially impressed, bounded by a slender, elevated 

 line, with the intervening area minutely wrinkled; pallial sinus rounded, ex- 

 tending in front of the vertical of the beaks; hinge concentrated, the anterior 



