254 MISSISSIPPIAN BRACHIOPODA 



in the genotype ; internally the socket plates are supported at their inner 

 margins by a pair of lamellae which pass obliquely towards the floor of the 

 valve to which they are joined near the median line, between these 

 lameike, the outer walls of the valve, and the socket plates, are a pair 

 of cavities narrowly triangular in cross-section, which expand anteriorly 

 and open out into the general cavity of the valve; the crura originate 

 from the anterior extension of the inner walls of the socket plates. 





FIG. 27. A series of seven cross-sections of the rostral portion of the shell of 

 DieZasmoides ftisinwata (X 2 1 /), showing the dental lamellae of the pedicle 

 valve, and the lamellae supporting the hinge-plate in the brachial valve. 



Remarks. In the presence of the triangular cavities in the apical 

 portion of the brachial valve this genus resembles Dielasma, but in this 

 case the outer bounding walls of the cavities are the socket plates rather 

 than the basal portions of the special crural lamellae, and the crura are an- 

 terior extensions from the inner walls of the sockets instead of arising as 

 separate crural lamellae originating at the apex of the valve. The special, 

 transverse, muscle-bearing plate is also absent in this genus. The genus 

 is perhaps to be compared with Girtyella as the form to which it is most 

 closely allied. As in Girtyella the crura originate from the inner walls 

 of the sockets, but the hinge-plate supported by a median septum is 

 wanting. The supporting lamella? of the socket-plates, however, may be 

 considered as a hinge-plate similar to that of Girtyella, which has become 

 so deeply concave that it rests upon and is coalescent with the outer wall 

 of the valve at and on either side of the median line. This arrangement 

 of the lamellse in the rostral portion of the brachial valve is considered 

 'as the essential generic character of the group; whether the bisinuate 

 configuration of the pedicle valve is also of generic value can not be 

 determined from the single species of the genus so far recognized. 



DlELASMOIDES BISINUATA Weller 



Plate XXXI, Figs. 19-24 

 1911. Dielasmoides bisinuata Weller, Jour. Geol., vol. 19, p. 443, figs. 3a-g 



Descrip Hon. Shell below miedium size, ovate in outline, 'the greatest 

 width anterior to the mid-length, the anterior margin rounded. The 

 dimensions of two nearly complete specimens are: length of pedicle 

 valve 20.5 mm. and 16.3 mm., length of brachial valve 18.9 mm. and 14.9 

 mm., greatest width 14.3 mm. and 12.8 mm., thickness 8.6 mm. and 8 mm. 



