98 TERTIAEY INSECTS OF NOKTH AMERICA. 



These buccal plates, as already stated, are the only hard parts of the 

 head, and the only appendages. Indeed, the only claim this portion of the 

 body has to be called the head at all is that it is certainly the anterior ex- 

 tremity of the digestive canal. On account of this peculiarity of the organi- 

 zation of the head, the creature, which is certainly widely different from 

 anything known, may be called Planocephalus (^TrXavccGo, uegjaXt^), and on 

 account of its onisciform body, Planocephalus aselloides. 



The first impression the sight of this strange headless creature conveys 

 is that of an isopod crustacean. But the limited number of legs at once puts 

 its reference to the Crustacea out of question, since no abdominal legs at 

 all are present. Even in the parasitic Crustacea, where some of the legs 

 are aborted, the same is the case with the segments themselves and witli the 

 joints of the legs which remain. Tiie clear distinction which obtains between 

 the thoracic and abdominal regions, and the limitation of the jointed legs to 

 a single pair on each thoracic segment seems to lead one strongly to tlie 

 conviction that these important elements of its construction place it among 

 insects. The structure of the legs and the small tapering abdomen furnished 

 with small anal appendages tend to the same conclusion. 



Where among insects it should be placed is more questionable. Think- 

 ing it possibly a larval form, careful search has been made among all the 

 groups into which it could by any possibility be presumed to fall, viz, among 

 the Neuroptera and Coleoptera, but nothing in the slightest degree seeming 

 to be related to it could be found, and its conspicuous size rendered it the 

 less probable that a kindred form would be overlooked On account, how- 

 ever, of its apterous chai'acter, and the discovery in recent years of certain 

 curious types of animals (all of them, however, very minute), whose affini- 

 ties have provoked more than usual discussion, my attention was early 

 drawn toward certain resemblances which Planocephalus bears to the Pau- 

 ropida among Myriapods and to the Tliysanura, and here, if anywhere, its 

 affinities seem likely to be found. 



Its passing resemblance to the obtected forms of Pauropoda whicli 

 Ryder has published under the name of Eurypauropodidfe is certainly very 

 considerable, especially when it is remembered that the young of Pauropoda 

 bear only three pairs of legs. The position of the more mobile part of the 

 head of Eurypaui-opus beneath the cephalic shield is the same that the head 

 of Planocephalus bears to the first thoracic shield ; and the mouth parts in 



