134 Ca7i O. Dunbar, 



linguiform extension, sharply deflected at ri^ht ang'les to the 

 valve. Dorsal valve very deep, sharply deflected at the margins 

 into the deep, nearly vertical sides, highest in front, and sloping- 

 with depressed convexity to the beak. Beaks small, and postero- 

 lateral sides often concave. Surface marked by rounded plica- 

 tions, of which three or four occupy the sinus, four or five the 

 fold, and five or six each lateral slope. Anteriorly the plications 

 are marked with a fine median line. 



Diinciisio)is: Length, 15 mm.; width, 14 mm.; height, 15.5 

 mm. A broad specimen measures; Length, 15 mm.; width. 16.8 

 mm.; height. 16.5 mm. 



Discussion: This species resembles U. abrupfiis and U. nuclco- 

 latus, but differs from either in the flatness of its ventral valve 

 (especially the umbonal region), in the slightness of its fold and 

 sinus, and the smaller number of plications on the fold and sinus. 



Occurrence: Common at most exposures of the Birdsong shale 

 and the Ross limestone. 



Eatonia tennesseensis, n. sp. 

 Plate III, figs. 9-1 1 



Description: Shell ovate to subtriangular. Ventral valve 

 depressed convex in the middle near the beak, but gently concave 

 between this point and the deflected margins ; below the middle 

 extended into a broad deep sinus which' bends upward in front. 

 in the more gibbous forms coming to lie almost at right angles 

 to the plane of the valve. A sharp angular inflection of the 

 posterior and lateral margins makes a distinct false cardinal area 

 which extends to the anterolateral corners. Beak small and 

 perforate, tightly incurved over that of the opposite valve so as 

 to hide the deltidial plates which cover the delthyrium. The 

 pedicle foramen is resorbed through the apex of the valve. 

 Dorsal valve gibbous, elevated anteriorly into a fold which cor- 

 responds to the sinus of the opposite valve. The posterolateral 

 margins are gently inflected so as to form with the inflected 

 margin of the opposite valve a concave space extending from the 

 beak along each side of the shell. 



In the interior the ventral muscular impression is large, 

 elongate oval, sharply defined by an elevated border, and divided 

 by a very low rounded septum. Just posterior to its middle are 



