ON VARIATION. 65 



the sixth form the ossification is extended so as to roof 

 in the temporal muscles and inclose the orbits behind, 

 while in the rare seventh and last stage, the tympanum 

 is also inclosed behind by bone. Now all of these 

 types are not found in all of the families of the Salien- 

 tia, but the greater number of them are. Six principal 

 families, four of which belong to the Arcifera, are 

 named in the diagram below, and three or four others 

 might have been added. I do not give the names of 

 the genera which are defined as above described, re- 

 ferring to the explanation of the cuts for them, but in- 

 dicate them by the numbers attached in the plate, 

 which correspond to those of the definitions above 

 given. A zero mark signifies the absence or non-dis- 

 covery of a generic type. 



It is evident, from what has preceded, that a per- 

 fecting of the shoulder-girdle in any of the species of 

 the arciferous columns would place it in the series of 

 Firjnisternia. An accession of teeth in a species of the 

 division Bt/fofiidcBv^oxAd. make it one of the Scaphiopid(Z\ 

 while a small amount of change in the ossification of 

 the bones of the skull would transfer a species from 

 one to another of the generic stations represented by 

 the numbers of the columns from one to seven. 



