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TA.'N'N JEtJJS has made three principal or grand Divisions of Tes- 

 tacea, viz. Multi valves, Bivalves, and Univalves and the following are the 

 generic description : — 



1. MULTI VALVES. 



1. CHITON : Valves placed in transverse plaits down the back. 



2. LEPAS : Valves unequal body sessile. 



3. PHOLAS : Shell bivalve with accessary valves at the hinge. 



2. BIVALVES. 



4. MYA : Hinge with generally a broad thick tooth not let into the oppo- 



site valve. 



5. SOLEN : Shell open at each end, hinge with a single or double subu- 

 late reflected tooth not let into the opposite valve. 



6. TELLINA : Hinge with the lateral teeth of one valve not let into the 



other. 



7. CARDIUM : Hinge with remote penetrating lateral teeth. 



8. MACTRA : Hinge with a complicated triangular middle tooth and an 



adjoining hollow. 



9. DON AX : Hinge with a lateral tooth generally remote not let into the 



opposite valve. 



10. VENUS : Hinge with generally three approximate divaricated teeth. 



11. SPONDYLUS : Hinge with two teeth, separated by a small hollow. 



