OF CONCHOLOGY. 313 



Reference to Plate 18. 



Figure C represents the central tooth and one complete series 

 of laterals on the upper line ; the lower line gives only the fourth 

 to the ninth lateral with the first marginal. 



Figure A represents a group of teeth to illustrate the merging 

 of the laterals into the marginals. The numerals on this and 

 the other figures give the count of the teeth from the central line. 



Figure E gives one line of teeth from the sixth lateral to 

 within two teeth of the extreme edge of the membrane. It illus- 

 trates the passage from the laterals through the five intermediate 

 marginals into the eight extreme marginals, as described above. 



Figure B shows several of the outer marginals from another 

 portion of the membrane. 



All the figures mentioned above are from the same membrane. 

 Fig. D is drawn from another membrane. It gives portions of 

 two lines of teeth, showing centrals and most of the laterals. 

 These last owe, perhaps, their appearance to hard usage or acci- 

 dent, but, as already observed, there is great variety in the re- 

 curved apex of the laterals. 



22 



