90 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



belong, and there are some special thysauuran features in its structure, anomalous as it is. Since 

 Packard lias sbown the reasonableness of placing the Symphyla ( = Scolopendrella) of Ryder in the 

 Thysanura, with theCollembolaand Cinura as co-ordinate 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 consid- 

 ering the relations of Planocephalus to Thysanura as a whole, to limit ourselves to points of com- 

 parison 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 tlioracic segments remind us 

 not a little of some Cinura, while tlie abdomen as a whole recalls many of the Collembola, its 

 approximated pair of specialized anal ai)pendages being also like the variously developed organs 

 of all Thysanura and unlike anything we can recall in any niyriapod. The legs, in the develop- 

 ment of the basal Joints and in the smaller double jointed tarsus, are ciosely related to those of 

 some Cinura — built indeed upon the same general pattern, excepting that in Planocephalus they 

 are specially developed for swimmjng. 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 obliteration of the cephalic 

 plates renders our fossil very distinct from any known type of Thysanura. But these features 

 separate it quite as widely from any other group that may be suggested for it, aud taking into 

 account the considerable develoi)ment of the thoracic portions, we must look upon Planocephalus 

 as in some sense a degraded form, descended from a type in which the head was developed at least 

 to some extent; and this renders it more probable that we have here found its proper place. More- 

 over when we examine the mouth-parts of Podura, we find them partially withdrawn within the 

 bead, 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 tliey now enjoy under the cephalic dome, imagine 

 further 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 Planocephalus 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 relations are certainly with 

 the Thysanura rather than with any other gruui); while tlie character of the legs, the half devel- 

 oped double claw, aud the anal appendages specialized to peculiar use are characters which are 

 positively thysauuran. Add to this that we find in Podura something in a remote degree analogous 

 to the extraordinary mouth-parts of Planocephalus, which we should in vain seek elsewhere, and 

 the probability that we find here its nearest allies is rendered very strong; and the more so from 

 the diversity of form aud type in this group since the addition to it of Scolopendrella. The dis- 

 covery of a co'lophore or something homologous to it would, we conceive, be decisive on the point; 

 but the lateral preservation of nearly all the specimens of this fossil, aud the obscurity of the base 

 of the abdomen in nearly all, not only forbid its determination in those yet found, but render it 

 doubtful if it will ever be discovered. 



The position of this g.roup among the Thysanura must be an independent one, between the 

 Cinura and the Symphyla, and of an equivalent value to them. For such a group the name of 

 Ballostoma is proposed, in reference to the remarkable power it possessed of thrusting forward the 

 gullet and mouth-parts. It would be characterized by the peculiarity named, by the lack of any 

 chitinous framework of the head, the equal development of three thoracic segments developed 

 dorsally as shields, and all separated from a cylindrical abdomen, which is armed at tip with a pair 

 of hooks for crawling; legs largely developed and with expanded and fiattened femora and tibire, 

 the tarsi two jointed. The principal points toward which attention should be dii-ected for the more 

 perfect elucidation of its structure are the buccal plates aud a possible collophore. 



