188 



ZOOLOGY 



it is now, for the great aboriginal shell-heaps (*' Kitchen- 

 middens ") of the New England coast contain shells of 

 enormous size. The cause of this gradual extinction of 

 the oyster is uncertain, but is believed to be due partly to 

 general climatic and geographic changes and partly to 

 over-iishiiip\ 



o 



APPENDIX TO CHAPTER XII 



KEY TO THE PRINCIPAL FAMILIES OF MARINE LAMELLIBKAN- 

 CHIATA OF THE EAST COAST OF THE UNITED STATES 



ADAPTED FKOM A. C. APUAK 



a\. Shell when closed at ventral side, gaping inure or 



less at the ends. 



bi. Without toothed hinge or proper ligament, 

 often with accessory valves ; no distinct epi- 

 dermis. 



c'i. Length decidedly greater than height 

 Co. Length and height nearly equal ; shell 

 small ....... 



b-2- With proper hinge, often distinctly toothed 



and with hinge ligament or pad. 

 t*i. Hinge with many small cardinal teeth in 

 each valve . . 



c-2. Hinge of not over 4 cardinal teeth in 



each valve. 

 di. Shells equivalve. 



e\. Two or more times as long as 

 high, gaping more or less at 



ends. 

 /i. Length '> to-0 times height ; 



epidermis polished ; um- 

 bones not over one-quar- 

 ter from anterior end 



Pholadidcc 



7.eredid(e 



(Shipworms) 



Ledidce 



(Ex. Y old in) 



Solenidce 



(Uazor Clatus) 



