320 THE GEOLOGICAL RECORD [CH. XVI 



geological record in placing these groups as the most archaic of 

 present-day Odonata. So that here, at any rate, we are on safe 

 ground. 



Again, the value of the Florissant beds for us lies in their 

 undoubted demonstration of the descent of the Agrionidae from 

 more complexly veined forms, with a larger prenodal area and 

 less petiolation of the wing-base. As our studies in the ontogeny 

 and morphology of this family point with ever-increasing certainty 

 in the same direction, we may dismiss for ever the old idea that 

 a simple venation was necessarily archaic. We shall thus learn 

 to see, in the Agrionidae, the highly specialized and excessively 

 reduced descendants of Calopterygid-like ancestors, which though 

 by the very opposite means, it is true have attained to a success 

 only comparable with that of the Libellulinae of to-day, and which 

 far outnumber all other existing families. 



The attached phylogenetic diagram (fig. 165) is an attempt 

 to reconstruct the phylogeny of the Odonata from the available 

 evidence. A dotted line indicates that there is no direct evidence 

 for the supposed line of descent. Where definite evidence is to 

 hand, the line of descent is represented by a continuous black 

 line, whose thickness is made to vary somewhat in proportion 

 to the relative abundance of species in each group. Thus, at the 

 present day, the Libellulinae and Agrioninae are the two dominant 

 groups, and lie at the two opposite extremes of menogenetic and 

 asthenogenetic specialization respectively. Hence, in the diagram, 

 they not only have the thickest lines of descent, but are placed 

 farthest apart. 



