OF THE PAST. 



209 



externomedian, internomedian, and anal The general disposition 

 of these veins is as follows: The mediastinal and scapular veins, 

 with their branches, which are superior (i.e., part from the main 

 vein on the upper or anterior side), terminate upon the anterior 

 margin. The internomedian and anal take the opposite course, 

 and their branches are inferior, or, at least, directed toward the 

 inner margin ; while the externomedian, interposed between 

 these two sets, terminates at the tip of the wing, and branches 

 indifferentlv on either side. 



.xterno* 



Fig. 119. Schematic view of Wing of Palaeozoic Cockroach, showing the 



veins and areas. 



Now these veins are all present in both front and hind wings 

 of pakeozoic Cockroaches, and also in the hind wings of existing 

 species ; but in the front wings or tegmina of the latter the 

 number is never complete, the externomedian vein being always 

 amalgamated either with the scapular, or with the interno- 

 median, and the mediastinal frequently blended with the 

 scapular vein. 



The hind wings are thus shown to be conservative elements 

 of structure, since they have preserved from the highest 

 antiquity both their transparency and their normal number of 



