224 Sellards — Structure of Paleozoic Cockroaches, 



of locomotion. The pronotum was as characteristic a feature 

 of the Carboniferous as of the recent cockroaches, and formed 

 quite as secure a protection for the small inflexed head. The 

 front wing had a similar arched form, and the anal area was 

 as well defined. Nevertheless, a closer inspection of the suc- 

 cessive types reveals the fact that the group has by no means 

 remained stationary throughout its long existence, bnt, like other 

 organisms, is subject to the laws of advance and specialization. 

 As will be gathered from what has preceded, the most marked 

 changes from paleozoic to recent times have been in the struc- 

 ture of the framew T ork of the front and hind wings, as w T ell as 

 in the abdomen and the ovipositors, and are thus in accordance 

 with the general rule of change from the simple to the more 

 complex, or from the generalized to the more specialized condi- 

 tion. In the front wings the tendency has been toward a 

 reduction of the main veins by fusion of one or more of them. 

 The main veins of both wings have approached more closely 

 to the costal border. These changes have been accompanied 

 by a less uniform development of the main veins and their 

 branching systems. The hind wing has acquired a longitudi- 

 nal, and in a few genera a transverse fold, and in most genera 

 a fan-like plaiting of the expanded anal area. Numerous and 

 comparatively strong cross veins, rare in the front wings, and 

 unknown in the hind wings of paleozoic forms, have now 

 become very commonly developed in both wings. Not only 

 have both wings departed more widely from the primitive 

 type, but differentiation between the front and hind wings has 

 increased as well.* The front wings have become, as a rule, 

 more resistant, although there were species in the Carbonifer- 

 ous with wings more opaque than some of the thin-winged 

 living species. Important changes have occurred in the abdo- 

 men. The terga and sterna have been modified, tending 

 toward a reduction in the number of abdominal segments. 

 The genital pouch has been perfected, and the ovipositors have 

 become reduced and adapted to perform a specialized function. 

 The division into coordinate groups, based originally on the dif- 

 ferences in the venation of the front wings, was strengthened by 

 the discovery of a well-developed ovipositor in paleozoic forms. 

 More complete knowledge of the second pair of wings brings 

 out additional distinctive characters. In the meantime, forms 

 more or less intermediate are coming to light, and it may be 

 confidently expected that the late paleozoic and early mesozoic 

 will in time yield other intermediate forms. The exact point 

 of disappearance of the Paleoblattidse and origin of the Blat- 

 tidse is at present unknown. The geological age of the Fair- 

 play deposits in South Park, Colorado, which contain the latest 

 forms of the one in association with the earliest of the other, 



*See also, Scudder, Mem. Boston Soc. Nat. Hist., vol. iii, p. 31, 1885. 



