506 S. WELLER MISSISSIPPIAN RHYNCHONELLIFORM SHELLS 



meet in the median line ; the two portions of the cavity thus formed are 

 of unequal size ; the one toward the cardinal side is smaller, subcrescentic 

 in form, and is completely arched over, as was the undivided cavity pos- 

 teriorly. The arched covering of the smaller crescentic cavity quickly 

 disappears and the slitlike, median, cardinal incision of the crural cavity 

 opens freely toward the interior of the shell (figures 5j, Tcyl). The lat- 

 eral, oblique cavities on each side of the central crural are inclosed upon 

 the cardinal side but for a short distance (figures bh, i, j), but the inner 

 walls continue as supporting plates abutting against the outer surface of 

 the walls of the crural cavity internally and against the inner surface of 

 the lateral walls of the valve externally (figures 5k, I), and they continue 

 beyond the anterior extremity of the crural cavity, giving support to the 

 crural bases (figures 5m, n). The median septum continues well toward 

 the anterior extremity of the valve with gradually decreasing height. 



In the pedicle valve of this species the dental lamellae unite to form a 

 spondylium which rests directly on the floor of the valve, and each plate 

 is supported by a lateral buttress plate connecting the outer surfaces of 

 the dental lamellae with the inner surfaces of the lateral walls of the 

 valve, these structures being essentially as in T. subcuneata, although the 

 buttress plates are situated nearer the cardinal margins of the valves, and 

 they slope toward the cardinal margin in passing from the inner to their 

 outer extremities. Shortly before the inner margins of the dental lamellae 

 become free from the cardinal margins of the valves these buttress plates 

 cease to reach to the inner surfaces of the lateral walls of the valve, and 

 continue as gradually decreasing processes on the outer surfaces of the 

 dental lamellae (figures 5n, o, p). 



In the internal structure of its pedicle valve this species is essentially 

 like T. siibcimeaia, and it is therefore placed with it in the genus Tetra- 

 camera, but the structure of the brachial valve is unique among all the 

 species examined, and it is possible that these characters should be con- 

 sidered as of sufficient importance to justify tlie establishment of a dis- 

 tinct genus for the species. 



RhYNCHOTETRA N. GEN". 



RHYNCHOTETRA CAPUT-TESTUDINIS (WHITE) 



Another rhynchonelliform shell referred to the genus Camarophoria 

 by Hall and Clarke, apparently because of the presence of a median sep- 

 tum in each valve, is Rhynchonella caput-testudinis White. This species 

 has a peculiar external configuration, differing markedly from all other 

 rhynchonelliform shells of the Mississippian faunas in its elongate, trian- 



