TETRACAMERA 



219 



simple plications. The dimensions of a nearly complete but somewhat 

 distorted individual are : length 32.5 mm., width 37 mm., thickness 27 mm. 

 Pedicle valve flattened on the umbo and in front of the umbo, gently 

 convex from the beak to the front in the median portion, abruptly de- 

 flected near the margin, the front of the shell being nearly at right angle 

 to the flattened umbonal region, towards the antero-lateral margins the 

 surface is at first elevated by a gentle concavity and is then abruptly de- 

 flected towards the opposite valve forming a sort of angular crest along 

 each antero-lateral margin, the postero-lateral margins are nearly straight 

 and meet at the beak at an obtuse angle of about 110 degrees, along these 

 margins the surface is abruptly bent towards the opposite valve, and is 

 usually somewhat inflected forming with a similar area on the opposite 

 valve, an obscurely plicated, flattened or concave area on each side of the 

 beak; mesial sinus broad, shallow, ill-defined, becoming somewhat deeper 

 anteriorly ; the beak small, pointed and moderately incurved ; the plica- 

 tions are strong, simple, rounded or a little subangular, and are separated 

 by more angular furrows, from 14 to 20 are present on the valve, several 



FIG. 17. A series of sixteen cross-sections of the rostral portion of the shell of 

 Tetracamera subtrigona (X l 1 ^), showing the characters of the internal 

 lamellae. 



next the postero-lateral margins often being faint or nearly obsolete, 

 about six or seven occupy the mesial sinus, the margin of the valve is 

 deeply and acutely serrate, the projections being opposite the furrows 

 between the plications, the deepest and strongest serrations are in the 

 front of the shell, on each side they decrease in strength with the weaken- 

 ing of the plications. Internally a strong median septum extends from 

 the beak nearly to the line of deflection of the valve near its anterior 



