RHYNCHOTBTRA 



507 



gularly snbovate form, its coarse plications, the great depth of the valves 

 toward the beak, and the broad, concave lateral surfaces of the shell 

 toward the beak. Material for the proper examination of the internal 

 characters of a typical member of this species has not been available, but 

 specimens of at least a closely allied form, which may perhaps be a dis- 

 tinct species, differing only in the arrangement of the coarse plications of 

 the shell, have been investigated, and the series of cross-sections observed 

 is shown in figure 6. In this shell the cross-section of the apical portion 

 of the brachial valve is subtriangular in outline, and is divided by a 

 median septum with a crural cavity internally, this cavity being arched 

 over on its cardinal side (figure 6a). This condition persists to the be- 



88 



Figure 6. — Cross-sections of the rostral Portion of Rhynchotetra caput-testudinis 



(White) f 



Tlii.s series of nine cross-sections (X 2%) is from a specimen from the Burlington 



limestone of Missouri 



ginning of the articulation of the valves, with the cardinal surface be- 

 coming broader and flatter (figures 6&, c), but beyond this point the car- 

 dinal surface is open and the divided portion of the median septum forms 

 an open crural cavity (figures M, e). The outer surfaces of the processes 

 forming this cavity are at first connected laterally with the two portions 

 of the divided hinge-plate, but anteriorly this connection with the lateral 

 margins of the valves ceases (figure 6/), and shortly thereafter the con- 

 nection with the median septum is broken (figure 6^), the lateral walls 

 of the crural cavity passing into the bases of the crura. The median 

 septum continues well toward the front with gradually diminishing 

 height. In this valve there is no structure comparable with the crura- 

 lium as seen in the typical representatives of Camarophoria, the crural 



