NEUEOPTERA— THYSANURA— BALLOSTOMA. 99 



both are confined to a somewhat similar circuhir area ; there are no eyes in 

 either, and the legs terminate in a single curved claw. 



On the other hand, not only are antennae of a highly organized character 

 developed in Pauropoda, but the upper portion of the head carries a cephalic 

 shield as large and conspicuous as the others ; two pairs of legs are de- 

 veloped in the adult on every or nearly every segment of the body, and 

 always on the abdominal to the same extent as on the thoracic segments, no 

 abdomen being distinct from a thorax as in Planocephalus, but all the joints 

 of the body entirely similar ; the legs of the Pauropoda are formed on the 

 myriapodal type, consisting of cylindrical undifferentiated joints, while those 

 of Planocephalus are hexapodal in character, having a clearly defined femur 

 and tibia, and a two-jointed tarsus conspicuously smaller and shorter than 

 the preceding joints, of different form and apically spined. 



The closer, therefore, we compare these two types the less important 

 seem the points of resemblance and the more important the points of diverg- 

 ence between them ; foi- in the clear distinction of the thorax and abdomen, 

 the absence of abdominal legs, and the structure of the legs themselves — 

 fundamental features of its organization — Planocephalus clearly belongs to 

 the true hexapod type of insects. 



Its probable reference to the Thysanura may be defended on both 

 negative and positive grounds. There is no other group of hexapods to 

 which it could be considered as more likely to belong, and there are some 

 special thysanuran features in its structure, anomalous as it is. Since 

 Packard has shown the reasonableness of placing the Symphyla (=Scolo- 

 pendrella) of Ryder in the Thysanura, with the Collembola and Cinura as 

 coordinate groups, the range of the Thysanura has been extended, and as 

 a group of equivalent taxonomic value to the larger divisions of winged 

 insects it has seemed itself to gain a better ratio vivendi. It is not necessary, 

 therefore, in considering the relations of Planocephalus to Thysanura as a 

 whole, to limit ourselves to points of comparison which it may have to one 

 or another of its subordinate groups, but consider any points of resemblance 

 we may find to any of these groups indifferently. The thoracic segments 

 remind us not a little of some Cinura, while the abdomen as a whole recalls 

 many of the Collembola, its approximated pair of specialized anal append- 

 ages being also like the variously developed organs of all Thysanura, and 

 unlike anything we can recall in any myriapod. The legs, in the develop- 



