OPISTHOCOELOUS VERTEBRAE 97 



eluding the newts and salamanders among the 

 Urodela, and a number of genera of Anura of 

 peculiar habits belonging to separate families 

 and each containing a single species, namely, 

 Bombinator the European Fire-bellied toad, Pipa 

 the Surinam toad, Alytes the Midwife toad, and 

 Discoglossus, the Painted frog, a circum-Medit- 

 erranean batrachian. The opisthocoely of Pipa 

 is probably independent of that of the other 

 named Anura ; that of the named Anura collec- 

 tively is probably independent of salamandrine 

 opisthocoely ; and Batrachian opisthocoely as a 

 whole is independent of that of Lepidosteus. 



Somewhat analogous to the reversed articu- 

 lation of vertebrae described in the preceding 

 paragraph is the reversed spire of the shell in 

 Molluscs and the reversed pose of the body in 

 Flatfishes. These two latter cases of situs inver- 

 sus were quoted by Bateson as examples of 

 discontinuity in substantive variation, but they 

 must be mentioned again here as examples of 

 convergent variation. "In both of the groups 

 named, some species are normally right-handed, 

 others being normally left-handed, while as indi- 

 vidual variations reversed examples are found. 

 . . . The fact that the reversed condition may 

 become a character of an established race is 

 familiar in the case of Fusus antiquus. This 

 shell is found in abundance as a fossil of the 

 Norwich Crag, such specimens being normally 



