DESCKIPTION OF AN ARTICULATE OF DOUBTFUL RELATIONSHIP. 89 



consolidation form an oval rounded mass. How such a pair of plates, or compound plates, could 

 Lave subserved any purpose in tbe procuring of food, I cannot understand, but that such is their 

 iu)t uufrequent appearance, especially when seen through and protected by the thoracic shield of 

 the first segment, is nevertheless the fact. It is to be hoped that other specimens may set this 

 matter at rest. Those at hand allow no more definite statement than has been made. About 

 three-fourths of the specimens of this species show the buccal plates more or less distinctly. In all 

 but three they lie outside the body, usually at a distance from it of about half the length of tlie 

 first thoracic segment. In a fourth specimen they lie half protruding at the front edge of the body. 



These buccal plates, as already stated, are the only hard parts of the head, and the onlj- append- 

 ages. Indeed, the only claim this portion of the body has to be called the head at all is that it is 

 certainly the anterior extremity of the digestive canal. On account of this peculiarity of the 

 oi'ganizatiou of the head, the creature, which is certainly widely different from anything known, 

 may be called Planocephalus (TtXaydco, neqiaXij), and on account of its onisciform body, Piano- 

 cephalus aselloides. 



The first impression the sight of this strange headless creature conveys is that of an isopod 

 crustacean. But tbe 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 with the joints of 

 the legs which remain. The 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 the conviction that these important elements of its construction place it among 

 insects. Tbe 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. Thinking it possibly a larval 

 form, careful search has been made among all the groups into which it could by any possibility be 

 l)resumed to fall, viz, among the Neuroptera and Coleojitera, 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, however, of its apterous character, and 

 the discovery in recent years of certain (surious types of animals (all of them, however, very minute) 

 whose afflnities have provoked more than usual discussion, my attention was early drawn toward 

 certain resemblances which Planocephalus bears to the Pauropidie among Myriapods and to the 

 Tbysanura, and here, if anywhere, its aflflnities seem likely to be found. 



Its passing resemblance to the obtected forms of Pauropoda which Ryder has published under 

 the name of Eurypauropodidre 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 Eurypauropus beneath the cephalic shield is the same that the head of Planoceph- 

 alus bears to the first thoracic shield ; and the mouth-parts in both are confined to a somewhat 

 similar circular area; there are no eyes in either, and the legs terminate in a single curved claw. 



On the other hand, not only are antenufe of a highly organized character developed in Pau- 

 ropoda, but the upper portion of the head carries a cephalic shield as large and conspicuous as the 

 others ; two pairs of legs are developed 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 undlfterentiated 

 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 spiued. 



The closer, therefore, we compare these two types the less important seem the points of resem- 

 blance, and the more important the points of divergence between them; for in the clear distinction 

 of the thorax and abdomen, the absence of abdominal legs, and the structure of the legs them- 

 selves — fundamental features of its organization — Planocephalus clearly belongs to the true hex- 

 apod type of insects. 



Its probable reference to the Thysanura may be defended on both negative and positive 

 grounds. There is no other group of hesapods to which it could be considered as more likely to 

 S. Mis, 69 12 



