504 S. WELLER MISSISSIPPIAN RHYNCHONELLIFORM SHELLS 



This structure is totally different from that of Camarophoria, in which a 

 true cruralium is developed between the level of the hinge-plate and the 

 floor of the valve, and continues as far anteriorly or even beyond the base 

 of the median septum. The character of the median septum and crural 

 cavity in this species is essentially identical with that of Camarotoechia 

 and of several other undisputed rhynchonelloid genera. The species 

 possesses, in addition to the median septum, two lateral lamellae in the 

 brachial valve which originate on the floor of the valve on either side of 

 the base of the median septum and abut against the inner cardinal sur- 

 face of the valve at their opposite extremity (figures 4:C, d, e, f, g, h), 

 these lamellae, with the median septum, dividing the apical portion of the 

 valve into four compartments. Anteriorly these lateral septa support the 

 two lateral divisions of the hinge-plate directly opposite the hinge- 

 sockets, to a point beyond where the inner margins of the divisions of the 

 hinge-plate are supported by the divided median septum. These lateral 

 lamellae terminate abruptly, beyond the line of articulation of the hinge, 

 and disappear entirely while the median septum is still considerably 

 elevated. 



In the pedicle valve the dental lamellaB are strongly developed and 

 curve toward the median line as they approach the floor of the valve, 

 forming a spondylium which rests directly upon the inner surface of the 

 valve posteriorly, but which is raised upon a low median septum ante- 

 riorly. From the outer surface of each dental lamella a nearly horizontal 

 plate passes across each lateral rostral cavity, joining with the inner sur- 

 face of the outer shell wall. These lateral buttress plates have been ob- 

 served in only one other rhynchonelloid shell, R. sub trig ona M. & W., and 

 they are the two accessory supporting lamellae mentioned by Hall and 

 Clarke in their diagnosis of Camarophoria, but which are wholly wanting, 

 as has been shown, in the genotype of that genus, and are not known to be 

 associated with the characteristic brachial cruralium of that genus. The 

 peculiar association of characters in this species is believed to be suffi- 

 ciently distinct from other rhynchonelloid shells to be worthy of recogni- 

 tion as a distinct and as yet undefined generic type, for which the name 

 Tetracamera is herewith proposed. The genotype of this new genus is 

 T. subcuneata (Hall), and the only other species which can as yet be in- 

 cluded in it is T. suUrigona (M. & W.), which is next to be described. 



TETRACAMERA SUBTRIGONA (M. d W.) 



Tetracamera suh trig ona is another rhynchonelliform shell which was 

 placed in the genus Camarophoria by Hall and Clarke. This species is 

 much larger than T. subcuneata and is proportionally much broader. Its 



