NEUEOPTERA— THYSANURA— BALLOSTOMA. 



95 



others, also narrows and becomes more arched in front, so as to form a sort 

 of hood. The legs are very broad and compressed, and adapted to swim- 

 ming, which was apparently their use, as there would be no need of such 

 compression to crawl into chinks when the body is so mach arched. The}' 

 consist of a femur, tibia, and two tarsal joints, terminated by a single curved 

 claw. The femur is very large, subovate, inserted (presumably by a coxa) 

 in large cavities, those of opposite sides separated by their own width, and 

 situated a little behind the middle of each segment. The tibia is also very 

 large and subovate, but more elongated and squarer at the ends, being 

 about twice as long as broad, and fringed on the anterior edge by a row of 

 delicate hairs as long as the width of the joint. Of the two tarsal joints, the 



Fig. 1, dorsal view; Fiff. 2. lateral view: Fig. 3. transverse sectittnal view of Piano 

 cephalus aselloiiies from the Oligocene of Florissant, Colorado, reslored, and magnified 

 about sis diameters. 



Fig. 1. 



basal is a little the larger, being both longer and stouter. Each is. armed 

 at the tip internally with a tolerably stout spine of moderate length, and 

 together they ai-e a little longer than the tibia, much slenderer, and quad- 

 rate in form. The terminal claw is about half as long as the terminal joint. 

 The hind legs are somewhat stouter and the middle pair a little shorter 

 than the others ; but otherwise they closely resemble each other. 



The different segments of the thorax, as stated, are protected above by 

 the development of distinct chitinous plates, the lower edges of which are 

 clearly marked, and extend downward to the concealment, on a side view, 

 of the lower part of the body. The abdomen, however, seems to have 

 no such specialization of the integument of the upper surface. It is stout, 

 apparently well rounded transversely, and tapers to a produced but blunt 

 tip, which is armed with a pair of slightly recurved stout claws, two or 



