CHAPTER VI. 



TABULAR SCHEME, 



3 ETAINING the arrangement with which we began, we will put 

 v the bivalves first, and begin with the Pholadacea : 



Shell with dorsnl pieces and apophyses 

 Apophyses long 



Shell prickly all over Pholas, 160-163. 

 Shell prickly only at fore end and with cup at posterior 

 end Pholadidea, 164. 



Apophyses short Xylophaga? .165. 



Shell continued into a calcareous tube with long siphons, having 

 a pair of calcareous pallets near the end Teredo, 166-171. 



We can then tabulate the group as follows : 

 A. Pallial line not indented ; no siphons 



Shell inequivalve 



Equilateral or nearly so 

 With ears Pecten, 33-43. 



Without ears 



With orifice in lower valve Anomia, 9, 10. 

 Without orifice Ostrea, 32. 



Very oblique 



With teeth Avicula, 30. 

 Without teeth Pinna, 31. 



Shell equivalve 

 With ears- 

 Hinge toothless ; one muscular scar Lima, 44-48. 

 Hinge with teeth ; two muscular scars Limopsis, 

 17, 18. 



Without ears 



Hinge with many teeth 

 Hinge straight Area, 11-15. 



Hinge curved 



Shell round, teeth in two groups Pedunculns, 16. 

 Shell elongated at posterior end, teeth on each 



side nearly equal Leda, 5-8. 

 Shell trigonal, teeth much more numerous on 

 posterior side Nncula, 1-4. 

 66 



