502 S. WELLER MISSISSIPPIAN RHYNCHONELLIEORM SHELLS 



of the cruralium continues to the anterior margin of the hinge-plate, and 

 even beyond, where it supports the bases of the crura (figure 3/), but 

 beyond this point it is rapidly reduced and soon disappears. The crura- 

 lium is narrow, never attaining the width of that in the other two species 

 which have been described, but becomes gradually wider toward the front ; 

 it becomes highly elevated above the floor of the valve anteriorly and is 

 produced beyond the base of the median septum. The crura are strongly 

 curved toward the opposite valve soon after the disappearance of the sup- 

 porting median septum (figure 3g). In the pedicle valve the dental 

 plates are curved toward the median line of the valve and form a spon- 

 dylium which rests directly upon the floor of the valve posteriorly, but is 

 supported by a low median septum anteriorly; it is not continued so far 

 toward the front of the shell as is that of the brachial valve. 



QOQQ 



Figure 3. — Cross-sections of the rostral Portion of Camaroplioria explanata (McChes.) 



This series of eleven cross-sections (X 2%) is from a specimen from tlie Chester lime- 

 stone of Illinois 



The development of a true cruralium in the brachial valve is consid- 

 ered as the essential generic character of the three species just described. 

 This cruralium is fundamentally different from the cruralium-like crural 

 cavity l^etween the lateral portions ' of the divided or undivided hinge- 

 plate of many of the rhynchonelloid shells, which does not continue an- 

 teriorly beyond the hinge-plate and which does not support, along the 

 median line of its concave surface, a continuation of the median septum. 

 The criterion which has come to be used during late years for the placing 

 of a rhynchonelliform shell in the genus Camarophoria — that is, the 

 presence of a median septum in each valve — must give way to the charac- 

 ter of the cruralium of the brachial valve. Many rh3rQchonelloid shells 

 possess a distinct median septum in the brachial valve, and the fact that 

 the dental lamellae of the pedicle valve converge somewhat more rapidly 

 in some examples and become joined as they reach the floor of the valve, 

 or even before they reach it, and thus give rise to a median septum, is not 

 sufficient basis for placing them in a different family, and even in a differ- 



