CONTRIBUTIONS TO PALEONTOLOGY. 319 



ElCHWALDIA RETICULATA, HaLL. 

 Rhijiicho-nella (?) reticulata, Hall; Transactions of the Albany Institute, Vol. iv. p. 217, 1862. 



From the Niagara group at Waldron, Indiana. 



ElCHWALDIA GIBBOSA, N. S. 



Shell ventricose, rather above the medium size, sub-triangular or broadly 

 ovate in outline ; baso-lateral angles abruptly rounded, and the 

 anterior border straight or scarcely emarginate. Ventral valve 

 strongly convex in the umbonal region, becoming very slightly 

 depressed on the anterior part by a broad undefined sinus, which 

 extends one-half the width of the shell ; beak small, pointed and 

 strongly incurved over that of the opposite valve ; the denuded 

 surface of the beak small. Dorsal valve a little more gibbous than 

 the ventral, very prominent in the upper half; the anterior part 

 occupied by a broad undefined elevation which is barely distin- 

 guishable from the general convexity of the valve. Surface 

 reticulations very fine, about three or four in the space occupied 

 by one on the E. coralifem, and two or three to that of one on 

 E. reticulata. 



This species may be distinguished from either of the other species of 

 the genus by its greater gibbosity, even surface, and by the much finer 

 reticulations of the exterior structure. 



Geological Formation and Locality. — In rocks of the age of the Niagara 

 group, in Perry and Decatur counties, Tennessee. 



ElCHWALDIA CONCINNA, N. S. 



Shell small, sub-triangular in outline, a little wider than long, broadly 

 rounded in front, nearly straight in the middle, and more abruptly 

 rounded on the baso-lateral angles. Valves somewhat regularly 

 convex; the ventral valve a little more elevated than the opposite, 

 and having a broad undefined sinus on the anterior half of the shell ; 

 umbo prominent ; beak small and closely incurved, the nude space 

 longer than wide. Dorsal valve regularly convex, without visible 

 trace of mesial elevation. Surface reticulation fine ; pits elongate 

 in the direction of the length of the shell ; interspaces a little 

 flattened on the surface. 



