CHAPTER III. 



Linear Series — continued. 

 Vertebrae and Ribs. 



The Meristic Variations of the vertebral column constitute a 

 subject of some complexity. In considering them it must be 

 remembered that numerical change may be brought about in the 

 series of vertebrae by two different processes : first, by Variation in 

 the total number of segments composing the whole column, in which 

 case the variation is truly Meristic ; and secondly by Variation in 

 the number or ordinal position of the vertebra? comprised in one 

 or more regions of the column, not necessarily involving change in 

 the total number of segments forming the whole series, and in this 

 case the variation is Homceotic. Though Homceotic Variation is 

 often associated with change in the total number of segments, 

 from the nature of the case it is rarely possible in any given 

 instance to distinguish clearly whether such change has occurred 

 or not. This arises largely from the fact that while to find the 

 total number of vertebra? it is necessary to know the exact number 

 of caudal vertebra?, in many specimens these are incomplete, and 

 even if present their number cannot often be given with con- 

 fidence. For these reasons the chief interest of this section of the 

 facts arises in connexion with Homceotic Variation, and the modes 

 in which it occurs ; but it must be constantly borne in mind that 

 in almost any given case there may be Meristic Variation also, though 

 the evidence of this may be obscured. 



True Meristic Variation in Vertebrae and Ribs. 



I. Vertebrce. 



True Meristic Variation, that is to say, change in the total 

 number of segments composing the whole column, may neverthe- 

 less be plainly recognized in certain animals. Among some of the 



