212 MISSISSIPPIAN BRACHIOPODA 



in passing from the beak to the front margin, the one on each side nearest 

 to the postero-lateral deflection of the valve giving origin to four or 

 more plications from its outer side which are arched across the oval, 

 postero-lateral flattened portion of the valve. 



Brachial valve more convex than the pedicle, arched from the beak 

 to the anterior and antero-lateral margins, the greatest depth of the 

 valve near the middle, the postero-lateral margins sharply deflected 

 to meet the similar deflected portion of the opposite valve and helping 

 to form the oval, flattened areas on each side the rostral portion of the 

 shell; mesial fold obsolete posteriorly, obscure and poorly defined an- 

 teriorly; beak small, pointed, incurved beneath that of the opposite 

 valve; plications similar to those of the opposite valve, but with the 

 younger plications often added by intercalation instead of by bifurcation ; 

 at the postero-lateral margins plications similar to those of the opposite 

 valve are arched across the deflected portion of the valve. 



The minute surface markings of the shell consist of exceedingly fine 

 radiating stride which are crossed by fine concentric lines of growth. 

 Stronger lines of growth are sometimes present at intervals, more es- 

 pecially towards the anterior margin. 



Remarks. In general form, size and proportions this species resembles 

 R. caput-testudinis, but it differs from that species in being notably 

 thicker and especially in the conspicuous and more or less frequent 

 bifurcation or intercalation of plications; the presence of the strong 

 plications which arch across the oval postero-lateral regions of the shell 

 also distinguishes this species from R. caput-testudinis. 



Horizon. Burlington limestone. 



Genus TETRACAMERA Weller 



Description. Shell rhynchonelliform, small or of medium size, triangu- 

 larly subovate to subpentagonal in outline, the mesial fold and sinus 

 poorly developed or nearly obsolete except towards the front, the lateral 

 and anterior margins of both valves deflected so as to stand at nearly 

 right angles to the plane of the valves, both valves marked by simple, 

 subangular or rounded plications which reach to the beak. Internally 

 the pedicle valve possesses strongly developed dental lamella? which 

 curve towards the median line as they approach the floor of the valve, 

 and form a deep spondylium which rests upon the inner surface of the 

 valve or is slightly raised on a median septum anteriorly, from the ex- 

 ternal surface of each dental lamella a transverse buttress plate extends 

 across the intervening cavity to the inner surface of the outer shell wall. 

 In the rostral portion of the brachial valve a strong median septum 

 is present which is undivided at the apex of the valve, but before the 

 articulation of the shell is reached a median incision is formed internally 



