RHYNCHOPORA 231 



the delthyrium broadly triangular, nearly filled by the beak of the 

 opposite valve, communicating at its apex with the foramen which en- 

 croaches upon the beak of the valve ; plications rather coarse, rounded 

 or subangular, simple, originating at the beak, three or rarely four 



FIG. 21. A series of ten cross-sections of the rostral portion of the shell of 

 Rhynchopora pustulosa (X 2%), only the first two (A and B) with the 

 pedicle valve; sections C to J show the undivided hinge-plate, the median 

 septum, and covered crural cavity. 



occupying the mesial sinus, and from four to six each lateral slope, 

 those towards the postero-lateral margins becoming faint or almost 

 obsolete, the plications in the sinus marked by a slight median groove 

 for some distance back from the anterior margin. Internally the hinge- 

 teeth are supported by a pair of dental lamellge whose length is equal 

 to from one-fifth to one-fourth the length of the valve. 



Brachial valve much more convex than the pedicle, becoming gibbous 

 in front, the surface sloping with a long, regular, or an increasing curva- 

 ture, from the highest point to the beak, and with a short, usually more 

 convex curvature to the anterior margin, the lateral slopes strongly 

 convex from beak to front, the anterior slope more abrupt, and curving 

 abruptly to the lateral margins; mesial fold obsolete in the posterior 

 half of the valve and only slightly differentiated in front; the beak 

 incurved beneath that of the opposite valve; the plications similar in 

 form and number to those of the opposite valve. Internally the cardinal 

 process is absent and the hinge-plate is undivided; a conspicuous median 

 septum is present which extends one-third the length of the valve or 

 more from the beak, posteriorly the median septum is divided, the two 

 branches supporting the hinge-plate and forming with it a crural cavity, 

 triangular in cross section, which is open only towards the front, the 

 divided septum continues to the position of the articulation of the hinge, 

 beyond which the septum is free, the hinge-plate is continued anteriorly 

 in the crura. 



Aside from the plications the surface of both valves is marked by 

 concentric lines of growth which are of irregular strength and are 

 irregularly arranged, usually being inconspicuous. The shell structure 

 is finely and closely punctate. 



