Ulanchestcr Memoirs, Vol. v. {\gi\), No. ^. ii 



The secondary character of the edentulousness of 

 Anodon is shown by two facts : — 



{a) The recurrence of rudimentary teeth in young and 

 adult Anodons. 



{b) The occurrence of an almost perfectly graded 

 series in the American forms, which, formerly 

 included with the Uniones or Anodontidae, are 

 now mostly placed in an intermediate group," the 

 Lampsilidae. The first members of this series 

 have perfectly developed true Unionid teeth, the 

 last are truly edentulous. 



This loss of teeth occurs along two lines. 

 (<^) The series rtiyriopsis bialatus. 



Lainpsilis laevissivms. 

 „ purpuj'atus. 



Cristaria Jierculea. 

 ^British and many American Anodons. 



Here the cardinals pass from a flattened, modified 

 form to entire absence, — the laterals becoming more 

 gradually rudimentary. 



ij)) The series ^Lainpsilis alata. 



„ coinplanata. 



U. presstis. 

 kA. fragilis. 



Here the laterals disappear before the cardinals. 

 Thus the edentulous forms have a double origin, — the 

 British Anodons belonging to one series and the Magari- 

 tanas to a second. 



Alation is unknown in the British Uniones, but is 

 common in the American Lampsilidae (formerly classed 

 as Uniones), It is a remarkable fact that the development 

 of alation is associated with the loss of teeth. These 

 characters may be connected with habits or conditions 



