APPENDIX TO CHAPTER XII 



189 



/ 2 . Length about 3 times 



height ; umbones central, Psammobiidce 

 / 3 . Length less than 3 times 



height ; surface covered 



with radial ridges, chalky 



white, umbones near an- 

 terior end . . . Petricolidce 

 / 4 . Length about 2 times 



height; epidermis 



polished, with notched 



border extending beyond 



the edge of the thin shell, Solemyidce 

 / 4 . Shell with thick black 



epidermis ; external liga- 

 ment prominent and on 



shorter end of the shell, Saxicavidce 

 Length less than twice height, 

 /i. Shell somewhat triangular, 



with internal cartilage 



between deep triangular 



pits, similar in each 



valve 



f. 2 . 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^ Length decidedly greater than 



height. 



/i. 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 



Mactridce 

 (Hen Clams) 



Semelidw 



Pandoridce 



Myidw 



(Soft-shelled Clftins) 



