THE GROWTH OF GROUPS 143 



spines, one nasal and two lateral, which are part of the 

 dermal armature. The length of the nasal spine bears a 

 constant relation to the rest of the body. In all the 

 specimens it is nearly equal to half the diameter of the 

 eye. But the lateral spines are quite different in the two 

 types. The two characters may be described thus 



The Character of Order. 



The bony plates covering the whole body are arranged 

 symmetrically in a pattern. Upon the ventral surface, 

 this pattern is simple and easy to describe. In the centre 

 of the space which lies between the bases of the paired 

 fins is a conspicuous hexagonal plate which is surrounded 

 and touched by six other plates. Between the bases of 

 the smaller or pelvic fins are one or two pairs of plates, in 

 front of this are a few others. Close behind the mouth 

 is a curved row of four small plates. The lateral spines 

 but not the nasal must be considered along with the 

 plates, since every individual with plates arranged accord- 

 ing to the above pattern is also provided with spines of 

 the following type. These spines are large relatively to 

 the size of the body, and are armed with four conspicuous 

 spinelets. 



The Character of Disorder. 



The plates on the ventral side surface are very numerous 

 and small and are scattered separately upon the skin 

 according to no discernible pattern. The lateral spine is 

 very small, and is covered with numerous inconspicuous 

 points. 



In fifteen of the collection the plates are arranged in 

 the orderly manner : in six the arrangement is disorderly. 



