RHYNCHOTETRA 209 



Brachial valve slightly deeper than the pedicle but similar in form, 

 it is gently convex through the greater portion of its surface, being 

 abruptly deflected towards the opposite valve near the postero-lateral 

 margins, the deflected portion is a little broader than that of the opposite 

 valve and is continuous with it, the two together forming the subovate, 

 flattened or slightly concave lateral areas in the posterior portion of the 

 shell; mesial fold obscure or obsolete, when best developed it is but 

 slightly differentiated and is restricted to the extreme anterior portion 

 of the valve ; beak pointed, incurved beneath that of the opposite valve ; 

 plications similar in all respects to those of the opposite valve. 



Surl'aee of both valves, when well preserved, marked by fine, radiating 

 stria?, about three or four of which occupy the space of one millimeter. 



Remarks. The internal characters of this species have not been fully 

 investigated, but the strong dental lamellae of the pedicle valve joining 

 to form a spondylium-like cavity near the inner surface of the valve, 

 are clearly visible in one example with the rostral portion of that valve 

 broken, and in the brachial valve a well developed median septum is 

 clearly seen upon the slightly weathered surface. There is no reason to 

 doubt but that the arrangement of the internal lamellae agrees essentially 

 with that of R. missouriensis, which has been carefully sectioned. The 

 species agrees most closely with R. caput-testudinis, but it is somewhat 

 smaller, with much less convex valves, with a more subovate outline and 

 somewhat finer plications. It is clearly distinct from R. gibbosum by 

 reason of its less convex valves and its simple plications which usually 

 originate near the beak and continue to the margin without additions by 

 subdivision or intercalation. 



Horizon. Chouteau limestone. 



RHYNCHOTETRA ELONGATUM n. sp. 

 Plate XXVIII, Figs. 9-12 



Description. Shell below medium size, longer than wide, the greatest 

 width in front of the middle, the anterior portion of the shell sub- 

 semicircular in outline, the posterior portion acutely subcuneate. The 

 dimensions of a nearly perfect individual are : length of pedicle valve 27.2 

 mm., length of brachial valve 26.5 mm., width 18 mm., thickness 8.5 mm. 



Pedicle valve depressed convex, greatest convexity posterior to the 

 middle, nearly flat .anterior to the middle, the postero-lateral margins 

 abruptly deflected towards the opposite valve, the deflected portion on 

 each side forming one-half of a suboval, flattened or concave region 

 on each side of the beak; mesial sinus essentially obsolete; beak small, 

 pointed, scarcely incurved; the umbonal region with a median septum 

 internally; plications nine or ten in number, those in the middle of the 



