TRANSACTIONS OF WAGNER 

 878 



TERTIARY FAUNA OF FLORIDA 



the chondrophore, while the anterior qne is continued ventrally by a narrow 

 accessory lamella ; hinge-plate quite oblique, without buttresses to the 

 cardinals. 



The species of this group are rare tropical forms, a single species being 

 found in a fauna when present at all. Geologically they go back to the Oli- 

 gocene. 



Genus SPISTJLA Gray, 1838. 

 Dall, Nautilus, viii., pp. 26, 40, 1894. Type Mac tra solida (L.) Gray, 1847. 



Shell small, subequilateral, trigonal, with a thin epidermis, adjacent beaks, 

 and concentrically grooved dorsal areas ; pallial sinus small, rounded ; gape 

 obsolete ; valves convex ; ligament sagittate, set in a callous area close to the 

 dorsal margin and not set off from the chondrophore by any shelly ridge ; 

 dental armature normal, strong, not concentrated ; the opposed surfaces of 

 the laterals transversely grooved ; left cardinal small, prominent, with a small 

 posterior accessory lamella, the posterior ends of both projecting over the 

 chondrophore; right cardinal with the arms coalescent above, the anterior 

 arm close to the dorsal shell-margin ; hinge-plate thick and flatfish ; exterior 

 smooth or concentrically striated; the dorsal areas ill-defined. 



Subgenus HEMIMACTRA Swainson, 1840. 

 Dall, Nautilus, viii., p. 26, 1894. Type M. solidissima Dillwyn. 



Shell large, ovate-trigonal, with grooved laterals and rather concentrated 

 hinge; the dorsal areas are not grooved and the anterior arm of the right 

 cardinal is confluent with its ventral lamina ; cardinals markedly compressed. 



Hemimactra is a new-world type, for the most part, while the typical 

 Spisula is old world, especially European, in its recent distribution, though 

 represented in the American Tertiaries. 



Section Mactromeris Conrad, 1868. 

 Dall, Nautilus, viii., p. 26, 1894. Type M. polynyma Stm. 



Shell like Heinimactra, but the laterals smooth, the cardinals not com- 

 pressed, and the anterior arm of the right cardinal not confluent with the 

 ventral lamina. (See pi. 27, figs. 3, 7, 13, 16, 24.) 



This type is especially characteristic of northwest America. Mactro- 

 dcsma (pondcrosa) Conrad is merely an extremely ponderous rotund Mactro- 

 meris of Miocene age; Pscudocardium Gabb also seems nothing more than 

 an unusually heavy, short, and elevated species of Mactromeris ; Vclcda Con- 



