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, 

 Cymboplwra 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 Mactra cequorea and rcctiliiic- 

 aris, which seem to belong in the Mcsodcsiiiatida, the following species were 

 described at an early date from the Claibornian. M. parilis Conr. (+ pygnicea 

 Lea), M. decisa, 2Lnd prcetcniiis Conr., all of which belong to the genus Spisida, 

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

 type specimens. In other Eocene beds are Spisida albirupiana Harris from 

 White Bluff, Arkansas ; Spisida mississippiensis Conr. and 6'. fiincrata Conr. 

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

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

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

 hinge-plate of Ptcropsis papyria 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 Spistila in 

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

 the lateral lamiuEe 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 S. ovalis 

 Gould also retains this character. 



Section Mactromeris Conrad. 

 Spisula (Hemimaotra) dodona n. s. 

 Plate 27, Figures 7, 13, 25. 

 Oligocene 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, mesially 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 



