tktrapi.odon ii95 



Key to sprcirs of Trtraplodon. 



Shell vvitli very sharp, high posterior ridge, T. quadr Hater us. 

 Posterior ridge rounded or subangnlar. 



Shell rhomboid. T. crosscanus. 



Shell trapezoid. 



With mail}- zigzag-radial ridges. T. multisulcatus. 



Ridges few, wide and short. T. rettistis. 



Ridges strong extending half way over the disk, 



T. ovatus. 

 Ridges fine, shell concentrically sculptured, 



T. schonibergianns; cordatus. 

 Shell quadrate, nearly smooth, yellow. T. quadratus. 



Shell more or less triangular. 



Beaks nearly central. T. ecorinatus. 



Beaks anterior. 



Radial ribs strong. T. baro ; ambigiius. 



Radial ribs low. T. JHrnafiiis; pectinatus. 



Group of Tetraplodon ambiguus. 



Characters as in the genus. 



Tetraplodon ambiguus (Sowerby). 



Shell subtriangular. greatly inflated, solid, quite inequilateral; 

 beaks very full and high, their sculpture consisting of strong, 

 radial bars, which extend well over the disk, in some cases 

 to the base of the shell ; anterior end narrowed and rounded ; 

 base almost straight or curved ; dorsal slope suddenly and 

 obliquely truncate, often somewhat angular behind the liga- 

 ment ; posterior ridge very full and high, narrowly rounded, 

 ending in a blunt point at or near the base of the shell ; epider- 

 mis thick, brownish, dull, showing concentric growth lines; 

 teeth vertically striate : pseudocardinals split up. usually there 

 are two or three more prominent than the rest in each valve; 

 laterals single or double in the right valve, double in the left ; 

 beak cavities deep and rounded ; anterior scars small, deep ; 

 nacre whitish. 



