DESCRlPTiON OF AN EXTINCT PALEOZOIC INSECT. 57 



The sum of characters in Geracus undoubtedly point in the 

 direction of the Thysanura, especially if we include in this group 

 that remarkable insect which inhabited the ancient lake basin of 

 Florissant, descril)ed by Dr. Scudder under the name of Plano- 

 cephalus ascelloides.* (See figure below). 



The Thysanura are distinguished among insects by the loose 

 aggregation of the body rings, and the imperfect specialization 

 of the thorax and abdomen. As Dr. Scudder has well remarked, 

 they in this respect, more than other insects, approach the 

 Myriapods ; and like these are devoid of wings. 



The Podura or Springtails, a section of the Thysanura, are 

 notable for their compact bodies, especially the abdominal part, 

 and for the few rings of which the latter region is composed ; in 

 these respects they are constituted similarly to Geracus. 



But while the modern Spi'ing-Tails illustrate 

 some points of structure in this Acadian genus, 

 others are more strikingly paralleled by Dr. Scud- 

 der's extinct genus, Planocephalus. This remark- 

 able genus differs from other active insects in the 

 reduction of the cephalic region to a probosis with 

 masticatory plates at the orifice. The proboscis 

 was capable of being withdrawn under the thorax 

 or could be projected the full length of the body ; 



Fig. 4. 

 Planocephalus when thus projected the head and oral tube must 



Magts^^Restored. have presented a close resemblance to that of 



Geracus. 



In the thoracic region the resemblance was not so close, as 

 here the Acadian genus more nearly resembled the Podura and 

 the higher insects. 



The abdomen of Geracus by its compactness is like that of 

 Scudder's headless genus Planocephalus, but has two joints more, 

 and by this and by its form is near lotoma flumhea Packard,! 

 among the Thysanurans. 



* U. S. Geol. Surv. Vol. xiii. Tertiary insects of North America, S. H. Scudder, 

 p. 94, Figs 1-3, 1890. 



+ Guide to the Study of Insects, p. 620, PI. 10, Fig. 6 and 7, New York, 1880. 



