100 TERTIARY INSECTS OF NORTH AMERICA. 



ruent of the basal joints and in the smaller double-jointed tarsus, are closely 

 related to those of some Cinura built indeed upon the same general pattern, 

 excepting that in Plauocephalus they are specially developed for swimming. 

 In the claw of our fossil genus we have something decidedly thysanuriform. 

 We have heretofore spoken of the two tarsal joints as each armed apically 

 with an interior spine, but that of the final joint arises from the base of the 

 curving claw, and takes on more or less its direction, though only half as 

 long as it, causing it to resemble very closely the smaller digit of the claw 

 of both Collembola and Cinura, which is always inferior to the larger, and 

 not infrequently, as in Lepidocyrtus, etc., straight instead of curved 



Of course, the rudimentary character of the head and the entire obliter- 

 ation of the cephalic plates render our fossil very distinct from any known 

 type of Thysanura. But these features separate it quite as widely from tiny 

 other group that may be suggested for it, and, taking into account the con- 

 siderable development of the thoracic portions, we must look upon Plano- 

 cephalus as in some sense a lowly form, descended from a type in whirh 

 the head was developed at least to some extent, and this renders it more 

 probable that we have here found its proper place. Moreover when we ex- 

 amine the mouth-parts of Podura, we find them partially withdrawn within 

 the head, reduced in external presentation to a small circle at the end of a 

 conical protrusion of the under side of the head. Take away the cephalic 

 plates, withdraw the mouth-parts to the same protection of the first thoracic 

 segment which they now enjoy under the cephalic dome, imagine fur- 

 ther that the mouth-parts could be protruded to their original position when 

 covered by a cephalic shield, and we have about the same condition of 

 things we find in Planocephalus ; indeed the extensibility of the mouth- 

 parts beyond the thoracic shield seems quite what one might expect after 

 the loss of the hard parts of the head ; and the mouth-parts of Planocepha- 

 lus bear much the same relative position to the first thoracic shield which 

 those of Podura bear to the cephalic shield. 



Assuming, then, that Planocephalus is a true hexapod, its general rela- 

 tions are certainly with the Thysanura rather than with any other group ; 

 while the character of the legs, the half developed double claw, and the 

 anal appendages specialized to peculiar use are characters which are posi- 

 tively thysanuran. Add to this that we find in Podura something in a 

 remote degree analogous to the extraordinary mouth-parts of Planocephalus, 



