BY R. J. TILLYARD. 709 



whose behalf natural selection has been exercised to so full and 

 favourable extent that it can hold its own, with little further 

 variation, in the changing world around it, has, we may suppose, 

 existed until the present day. To us then it will appear as an 

 aberrancy, a form without near relations, and one whose origin 

 we can scarcely hope to explain, and whose position in a linear 

 classification it were almost hopeless to determine. 



Petalura is a good example of these aberrant forms. Its great 

 size and remarkable formation will make it one of the Odona- 

 tologist's chief treasures and most careful studies. Let us examine 

 carefully then the points which may throw light upon its position 

 in the recognised classification of Odonata : — 



1. Head rather small for the size of the insect, being scarcely 

 as broad as the thorax. In the Gom^^hince and Calopterygidm 

 the head is broader than the thorax, the nearest approach to 

 equalit}^ being in the genus Ictinus (a genus which I think shows 

 some points of approach to Petalura). As regards the eyes, these 

 are slightly smaller and further apart, considering the size of the 

 insect, than in the Gomjjhince, but larger and closer together than 

 in the C alopterygidoi. 



2. Thorax large and powerful. It shows a near approach to 

 the Gomphince. As regards the size of the prothorax, which is 

 fairly well developed, we see again a similarity to the Calopiery- 

 yidoi. The legs are strong and thick, but afford no criterion for 

 comparison; the Odonata on the whole use them but little, and 

 they show little variation. 



3. Abdomen: a point of great interest is the presence of 

 rudimentary spurs on the second segment of the male. The 

 Gomphince have well-developed spurs, the Calopterygidce have 

 none. In Petalura we have the spur about half-formed. As 

 regards the general shape of the abdomen, Petalura is widely 

 different from the Gomphince, and very similar to many of 

 the genera of the Calopterygidce, (compare for this purpose 

 Diphlehia), 



