Sellards — Structural Characters of Cockroaches. 313 



The fragment of a front wing from the same locality, men- 

 tioned by Scudder* " to draw attention to an unknown species 

 remarkable for its immense size " is probably of the same 

 species. 



The large hind wing also from Mazon Creek, described and 

 figured in the same memoir (1. c, p. 142), doubtless belongs to this 

 genus and species. Scuddersays of this wing : "A single speci- 

 men of a hind wing of enormous size differs so much from any 

 other known that I use for it, as a convenience merely, the old 

 generic term Blattina, as applied to fossil cockroaches in gen- 

 eral." The estimated length, 51 mm , is approximately the size 

 to be expected for the hind wing of a small individual of this 

 species. 



General Considerations. 



The most important differences between the Paleozoic and 

 recent cockroaches have already been indicated. Evolution 

 within the group, although not rapid considering the lapse of 

 time since the Paleozoic, has been progressive and directly in 

 the line of increased specialization and differentiation of the 

 organs affected. In both the front and the hind wings, there 

 has resulted a less equal development of the main veins. There 

 has also occurred in both wings, a migration of the main 

 trunks of the veins toward the costal border, with a consequent 

 reduction in the superior branches of these veins and the areu, 

 occupied by them, as will be seen by com paring the more 

 primitive Mylacridse with the Blattinarise. Cross veins, which 

 rarely occur in the front wings, and so far as known are absent 

 from the hind wings of Paleozoic forms, have since become 

 almost universal in both wings. 



Not only have both wings departed more widely from the 

 primitive type but differentiation between the front and hind 

 wing has increased as well.f The front wings have become, 

 as a rule, more resistant, although there were species in the 

 Carboniferous with wings more opaque than some of our liv- 

 ing thin-winged species. The hind wings have developed a 

 longitudinal fold and plaiting and an increased e ipanse of the 

 anal area.;}; 



Important changes have occurred in the abdomen. The 

 terga and sterna have become modified, tending towards a 

 reduction of the number of abdominal segmen ! cs. The genital 

 pouch has been perfected, and the ovipositors have become 

 reduced and adapted to perform a specialized function. 



On the other hand, there are certain broad resemblances 

 which should not be overlooked, implying as they do a close 



*Bnll. U. S. Geol. Surv., No. 124, p. 55, 1895. 



fSee also Scudder, Mem. Boston Soc. Nat. Hist., vol. iii, p. 320, 1885. 

 X A few recent genera have developed in addition to the longitudinal fold 

 a transverse fold of the hind wing. 



