122 Sellards — Structure of Paleozoic Cockroaches, 



representatives of modern Orthoptera and the early forms of 

 other orders is, even at that time, an actual one, indicating a 

 considerable divergence in the phylogenetic lines. On the 

 other hand, the apparent break between Carboniferous and 

 later Orthoptera is accidental and due to the fact that not all 

 of the intermediate forms have been preserved, or if preserved 

 have not yet been found. The object of schemes of classifica- 

 tion, as is universally conceded, is to express, as far as possi- 

 ble, phylogenetic relations. Following this leading principle, 

 any natural group of organisms should be recognized as 

 extending back in time until a point is reached at which that 

 group coalesces with a group or groups of coordinate rank, or 

 unites with the parent stock. It may often be a matter of 

 difficulty, owing to the imperfect geological record and other 

 causes, to determine the exact point of origin of a group from 

 an ancestral stock. In the case of the Orthoptera, how T ever, 

 the evidence at hand seems conclusive that the order as a dis- 

 tinct phylum is recognizable well into or beyond the Carbon- 

 iferous. Not only does the body structure of these early forms 

 present the essential features of the Orthoptera, but the 

 development, as is here shown, is in entire agreement with that 

 of modern forms, the young resembling the adults and growth 

 taking place by a succession of moults during which the wings 

 appear gradually. 



Super-Family Blattacea. 



The cockroaches of the Carboniferous, as shown by the fore- 

 going detailed account, are closely related to those of mesozoic 

 and later times. The early forms present not a few T generalized 

 characters, such, for example, as the protruding ovipositor, 

 lessening the interval separating them from other primitive 

 Orthoptera. Nevertheless they have already acquired many 

 of those peculiarities which distinguish them from all other 

 insects, notably the rounded shield-shaped pronotum, the small, 

 flat, retractile head, the characteristic strongly delimited anal 

 area of the front wing, the flat bodies, and, without doubt, the 

 peculiar scurrying motion. The indications are that their 

 habitat was much the same. Their constant association with 

 fossil plants, especially ferns, suggests that they were then, as 

 now, fond of moist low places, with abundant vegetation, 

 along the banks of rivers and marshes. Such differences as 

 are found are those resulting naturally from development and 

 specialization, as increased differentiation between the front and 

 hind wing, greater complexity of the wing venation, and the 

 reduction of the ovipositor by specialization. The indications 

 are that the line representing the cockroaches diverges from 

 that of other insects during the early Carboniferous or pos- 



