MESOZOIC COCKROACHES. 441 



and in fact very imperfect knowledge, this region being frequently missing in the fossils. 

 In most of the genera the anal nervnles, so far as known, strike the margin, but in some 

 the species vary in this respect; in others their course is entirely unknown, while in such 

 as are perfectly preserved in the most prolific genus, Mesoblattina Geinitz, they impinge 

 indeed upon the margin, but show a decided tendency to direct themselves toward the 

 tip of the anal furrow, as in many modern forms. This feature cannot therefore be said 

 to have become fairly established in mesozoic times. 



These changes in the general structure of the front wing are no doubt but one ex- 

 pression of the increasing heterogeneity in the neuration of the front and hind wing 

 winch was almost entirely unknown in paleozoic times, but which has reached a high 

 development at the present day. The remains of the hind wings of mesozoic cock- 

 roaches are indeed not sufficiently abundant to prove this, but we have grouped here 

 under one generic name, Aporoblattina, such single detached wings as seem to be prop- 

 erly considered as such, and here the veins are entirely distinct. Another indication of 

 this specialization on the part of the front wing is their increasing density, by which 

 the neuration is in part obscured. This is not very marked, but in some species is un- 

 mistakable. 



A further peculiarity of mesozoic species, as a general rule, is their small size. In a 

 previous paper, before the number of paleozoic forms known was as great as now, the 

 average length of their front wings was estimated as 26 mm., and there is no reason to 

 suppose that that measurement would be altered by later discoveries to any extent worth 

 mentioning. On the other hand, even the Triassic Palaeoblattariae already show a ten- 

 dency toward that diminution in size which is well marked in the mesozoic Blattariae, 

 for the average length of the former is only 16 mm., while in the mesozoic Blattariae as 

 a whole it is still further reduced to 12.5 mm. Even this would be somewhat diminished 

 (to 11.5 mm.) if we should omit the species from the middle Oolitic beds of Solenhofen, 

 all of which were large and some gigantic, one reaching a length of 60 mm. That this 

 should be the case seems a little unexpected when Ave find the species of the upper 

 Oolite (of England) a little smaller as a general rule than the Liassic forms. This 

 somewhat curious fact led me to ask what should be considered the average size of the 

 modern cockroach. I accordingly took Brunner's Systeme des Blattaires and tabulated 

 the measurements of the front wings given there whenever the material was at hand for 

 the purpose, to the number of 243 species. One measurement only was taken for each 

 species and where the sexes differed (as often excessively) these also were averaged. 

 Of course the apterous species had to be omitted, and it was plain that the result would 

 be too large as the larger species find their way to collections much more rapidly than 

 the smaller forms. The general result was that the average size of the front wings of 

 recent cockroaches is 18.2 mm. which is considerably more than that of the mesozoic spe- 

 cies, and much less than that of the paleozoic forms. 



As regards the relative geological position of these mesozoic cockroaches two facts 

 are patent: 1°. No species has been found in more than one deposit. 2". While all three 

 of the genera of the Trias are peculiar to it (some of the genera of the Triassic Palaeo- 

 blattariae have also been found in lower paleozoic rocks) and two genera are found only 

 in the upper Oolite, all of the genera found in the intermediate Lias also occur in the 



