As regards the other structural distinction between the _Palæoblattariæ and Neoblattariæ_--the course of the anal nervules--there is much diversity, and very imperfect knowledge, since this very portion of the wing is not infrequently lost, a fracture most readily occurring at the anal furrow. In most of the mesozoic genera, the anal nervules, as far as known, strike the margin; but the larger portion of these show a decided tendency to trend toward the tip of the anal furrow, as in many modern forms. This feature can hardly be considered as firmly established in mesozoic times, and the same genus, as _Scutinoblattina_, may contain species which differ in this respect. A further peculiarity of mesozoic Cockroaches, already alluded to, is their generally small size. The average length of the front wing of palæozoic Cockroaches has been estimated to be 26 mm., that of the Triassic _Palæoblattariæ_ is about 16 mm., while that of the mesozoic _Neoblattariæ_ is 12·5 mm. One exception to this small size must be noted in the species from the Jura of Solenhofen, all of which were large and some gigantic, one wing reaching the length of 60 mm., or about the size of our largest tropical _Blaberæ_. If we omit these exceptional forms, the average length of the wing of the mesozoic Cockroach would be scarcely more than 11 mm. Now an average of the 243 species of which the measurements are given in Brunner’s Système des Blattaires (1865), gives the length of the front wing of living Cockroaches as a little over 18 mm.; so that the mesozoic Cockroaches were as a rule considerably smaller, the palæozoic Cockroaches much larger, than the living. [Illustration: Fig. 123.--_Mesoblattina Brodiei_ Scudd. × 4. Purbecks, England.] Nearly eighty species of mesozoic _Neoblattariæ_ are known, and they are divided into thirteen genera,[198] one of which, _Mesoblattina_ (see figure of _M. Brodiei_), contains upwards of twenty species, mainly from the Lias and Oolites of England. The Upper Oolite has proved the most prolific, considerably more than half the species having been found in the English Purbecks, while nearly a fourth occur in the Lias of England, Switzerland, and Germany. Many of the English species have been figured in Brodie’s Fossil Insects of the Secondary Rocks of England, in Westwood’s paper on Fossil Insects in the tenth volume of the Quarterly Journal of the Geological Society, and in the memoir alluded to above. No species has yet been found in rocks of different geological horizons, and the genera of the Trias are peculiar to it. So, too, are some of the genera of the Oolite, but all of the Liassic genera occur also in the Oolite. [198] See a paper on mesozoic Cockroaches now printing in the Memoirs Bost. Soc. Nat. Hist., Vol. III., p. 439 seq.