274 MISSISSIPPIAN 3BACHIOPODA 



strongly incurved, coming nearly or quite in contact with the umbonal 

 region of the opposite valve, pierced by a large, oblique, subovate or 

 subelliptical foramen which encroaches upon the umbonal region, coming 

 in contact with the delthyrium only at its apex. Internally the dental 

 lamellae are well developed and extend anteriorly from the beak with but 

 slight divergence for about one-fourth the length of the valve. 



Braehial valve nearly equally convex with the pedicle, the greatest 

 convexity posterior to the middle, the umbo prominent, the surface 

 curving abruptly to the posterior and postero-lateral margins, the curva- 

 ture becoming more gentle to the aiitero-lateral and anterior margins; 



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FIG. 32. A series of seven cross-sections of the rostral portion of the brachial 

 valve of Girtyella turgida (X 2 1 /&), showing the median septum supporting the 

 hinge-plate, and the manner of formation of the crura. 



mesial portion of the valve depressed in a sinus originating in front of 

 the umbonal region, which is somewhat broader, shallower and flatter 

 than that of the pedicle valve, occasionally a slight, rounded rib occupies 

 the median line of the sinus towards the front; the beak pointed and 

 incurved beneath that of the opposite valve. Internally the socket plates 

 are joined by a concave hinge-plate which is supported by a median 

 septum, anteriorly the socket plates retreat from the lateral margins of 

 the valve but are still joined together by the hinge-plate and are joined 

 to the inner surface of the lateral slopes of the valve until they are 

 continued into the free bases of the crura; the brachidium short, not 

 reaching to the middle of the valve. 



Surface of both valves sometimes nearly smooth but usually marked 

 by concentric lines of growth which vary greatly in strength and dis- 

 tribution in different examples ; shell structure finely punctate. 



Remarks. This species has frequently been misinterpreted, and 

 three or more distinct species belonging to three different genera have 

 commonly been known under this name. So far as it has been observed 

 the species is always a small and rather gibbous form, but it varies greatly 

 in its gibbosity and also in the development of the concentric lines of 

 growth, the more gibbous examples commonly being most strongly 

 marked by growth lines. The species differs from G. indianensis, the type 

 of the genus Girtyella, in its shorter shell with more gibbous and usually 

 less smooth valves, and in the much more conspicuous sinus of the 

 brachial valve. 



Horizon. Salem limestone. 



