GENUS SEBASTODES. 581 



series of facts. When the three spines are present to- 

 gether, the distance from the base of the tympanic to the 

 base of the supraocular on the one hand, and the distance 

 from the supraocular to the preocular on the other hand, 

 are to each other in many species as i to i, varying from 

 this ratio to i to 3 in rosaceiis; while where one of the 

 spines is absent, the relative distances vary from 3 to 10 

 to 3 to 15 (except nebulosus, 2 to 5). These measure- 

 ments give the all but invariable rule that, when one of 

 the spines is absent the so-called supraocular occupies 

 the position of the postocular. When both the supra- 

 ocular and postocular are present and differ in size (which 

 is usually the case), the supraocular is invariably weaker 

 than the postocular. The depression between the tym- 

 panic and postocular is always deep, while between the 

 postocular and supraocular there is frequently a well- 

 marked ridge (chlorostictits, rhodochloris, riiherrimiis). In 

 levis the true supraocular is usually present; in the skull 

 at hand it was absent, but on one side a blunt knob occu- 

 pied the position required by the rule of relative distances, 

 and just behind this point, on both sides, there was a de- 

 pression in the otherwise continuous ridge, marking the 

 depr^ession between the supra- and postoculars. In the 

 skull of elongatus, in which one of the pairs of spines is 

 normally absent, there is a low, conical rudimentary spine 

 on the left side, occupying the position of the supraocular, 

 as required by the rule of relative distances. These 

 facts, taken together, seem to establish the conclusion 

 that when one of the trio of pairs of spines is absent, the 

 supraocular spine has disappeared, and the supraocular 

 ridge merged with the postocular. 



A source of error that had to be studiously avoided in 

 the comparison of species is that due to the changes that 

 take place with increasing age. Of these, the following 



