APPENDIX TO CHAPTER Xll 



189 



/:>. Shell thin, rounded in front, 

 narrowed and gaping be- 

 hind, cartilage pit shal- 

 low ; external ligament 

 short .... 

 Shells inequivalve. 

 e\. Length and height about equal, 

 e-2. Length decidedly greater than 



height. 



/!. Over 25 mm. long ; right 

 valve nearly flat ; whole 

 shell compressed to one- 

 sixth length . 



/ 2 . Over 50 mm. long ; 'slightly 

 inequivalve ; no external 

 ligament; internal car- 

 tilage on spoon -shaped 

 process of left valve 



f. 2 . Length about 3 times 



height ; umbones central, Psammoliidfc 

 f s . Length less than 3 times 



height ; surface covered 



with radial ridges, chalky 



white, umbones near an- 

 terior end 

 / 4 . Length about 2 times 



height; epidermis 



polished, with notched 



border extending beyond 



the edge of the thin shell, 

 /4. Shell with thick black 



epidermis ; external liga- 

 ment prominent and on 



shorter end of the shell, 

 e. Length less than twice height. 

 J\. Shell somewhat triangular, 



with internal cartilage 



between deep triangular 



pits, similar in each 



valve 



Solrmyidw 



SaxicctvhUv 



Mactridce 



(1 leu Clams) 



Pmidoriflre 



