586 SYSTEMATIC PALEONTOLOGY 



" Posterior adductor scar apparently small and situated near the dorsal 

 margin, at about the mid-length of the shell; a sharp crested ridge or 

 carina extends from a short distance behind this scar backward to about 

 the middle of the posterior extremity. Dorsal margin broadly concave ; 

 anterior margin broadly and regularly rounded; ventral margin regu- 

 larly rounded anteriorly, notched, the notches corresponding to the 

 interspaces between the ribs ; the broad curve carries the ventral margin 

 upward toward the high narrow posterior portion of the shell, where the 

 margin curves slightly downward becoming concave, and meeting the 

 posterior margin in a subright angle : posterior margin short, squarely 

 truncated, and situated above the mid-height of the shell. 



" Surface of the adult marked by 20-22 prominent ribs which originate 

 along the lower margin of the dorsal area and extend to the anterior and 

 ventral margins; the ribs on the anterior portion of the shell trend first 

 forward and downward, and then sweep in a gentle curve around to the 

 anterior margin; from the front toward the rear the ribs become suc- 

 cessively straighter, tending first downward, and, toward the posterior 

 extremity, backward and downward ; the crests of the ribs are poorly pre- 

 served but are apparently tuberculated. 



"Dimensions. Length 37 mm.; height 27 mm.; convexity 7 mm. 



"' This species differs from Trigonia eufaulensis Gabb in the closer 

 spacing and smaller degree of curvature of the ribs, and in the greater 

 curvature of the ribs, and in the greater elongation of the posterior portion 

 of the shell. The species is distinguished from the young of Trigonia 

 bartrami by the relatively closer spacing of the ribs and the greater pos- 

 terior elongation of the shell. 



"Type.U. S. National Museum, No. 31642. 



" Occurrence in South Carolina. Snow Hill member of Black Creek 

 formation (upper part of Exogyra ponderosa zone). Hodge's old mill 

 site, 3^ miles southeast of Mullins, Marion County. 



"' Occurrence in Alabama. Tombigbee sand member of Eutaw forma- 

 tion (lower part of Exogyra ponderosa zone). Seaboard Air Line Kail- 

 way at bridge over Hatchechubbee Creek, 2 miles west of Pittsview, 

 Russell County." Stephenson, MS. 



