TETRACAMERA 



505 



geologic horizon is the Keokuk limestone. The internal characters of the 

 shell are exhibited in the series of cross-sections shown in figure 5. The 

 rostral cavity of the brachial valve is divided by a median septum which 

 is at first solid (figure 5e), but is soon excavated on the median line near 

 the cardinal side of the valve. This median excavation is completely 

 arched over to the line of articulation of the valves (figures 5f, g, h, i), 

 beyond which it is open on the cardinal side by a narrow, median, slit- 

 like incision (figures 5j, h, I), thus forming a crural cavity which differs 

 from that of T. subcuneata in having a much greater anterior extension 

 and in having the median incision on the cardinal side much narrower; 



^f? 



ff^'^'S^gM 



Figure 5. — Cross-sections of the rostral Portion of Tetracamera suhtrigona (M. & W.) 



This series of sixteen cross-sections ( x 1 %= ) is from a specimen from ttie Keokuk lime- 

 stone of Keokuk, Iowa 



this structure, however, is totally different from that of the true cruralium 

 of Camarophoria. At the line of articulation of the valves a shallow, 

 broad, oblique cavity originates on each side of the central crural cavity 

 (figures 5h, i, j), whose bounding wall on the cardinal side is much thin- 

 ner than the inner wall. The inner walls may perhaps be compared with 

 the two lateral septa of the rostral portion of the brachial valve of T. 

 subcuneata, although their position is very different, their inner extremi- 

 ties abutting against the outer surface of the walls of the crural cavity 

 and their outer extremity against the inner surface of the sides of the 

 valve. Shortly after the origin of these lateral, oblique cavities, the 

 crural cavity is differentiated into two portions (figure 5{) by the out- 

 growth from its inner surface of a pair of lateral processes which nearly 



