TETRACAMERA 



213 



which is at first arched over but soon becomes open internally by the in- 

 cision between the lateral divisions of the divided hinge-plate, the crural 

 cavity so formed being essentially identical with that of Camarotoschia; 

 the cavities between the median septum and the outer shell walls are 

 each divided by a lamella or septum resting upon the floor of the valve 

 and supporting the inner margin of the corresponding lateral division 

 of the hinge-plate, these lateral septa extending farther anteriorly than 

 the median septum. 



FIG. 16. A series of ten cross-sections of the rostral portion of the shell of 

 Tetracamera subcuneata (X 2%>, showing the dental lamellae and buttress 

 plates of the pedicle valve, and the median and two lateral lamellae, the crural 

 cavity and crura of the brachial valve. 



Remarks. The species included in this genus were referred to Cam- 

 arophoria by Hall and Clarke, 1 and usually have been so referred since 

 the publication of their work. The shells do not possess the character- 

 istic eruralium of Camarophoria, besides possessing other characters 

 wholly foreign to that genus. The most essential generic characters are 

 the presence of the buttress plates connecting the dental lamella? with 

 the inner surface of the outer shell wall, and the presence of the supple- 

 mentary lamellae in the rostral portion of the brachial valve in addition 

 to the median septum. The species T. subtrigona which is here included 

 in the genus differs in a rather fundamental manner from the genotype, 

 not only in the general form of the shell but in the arrangement of the 

 internal lamella?, and should perhaps be made the type of a distinct 

 genus, but the discussion of these characters is reserved for treatment 

 under the specific description. 



iPal. N. Y., vol. 8, pt. 2, p. 214 (1894). 



