These differences, which were mentioned by Germar and Goldenberg, and their universality pointed out in my memoir on Palæozoic Cockroaches,[197] seem to warrant our separating the older forms from the modern as a family group, under the name of Palæoblattariæ; this family has been thus characterised:—
Fore wings diaphanous, generally reticulated, and nearly symmetrical on either side of a median line. Externomedian vein completely developed, forking in the outer half of the wing, its branches generally occupying the apical margin; internomedian area broad at base (beyond the anal area), rapidly tapering apically, and filled with oblique mostly parallel veins, having nearly the same direction as the anal veinlets, which, like them, strike the inner margin.
Fig. 120.—Etoblattina mazona Scudd. × 3. (The outline of natural size.) Carboniferous, Illinois.
About eighty palæozoic species have been published up to the present time, and have been grouped in two sub-families and thirteen genera. Besides these, Brongniart has not yet given any hint of how many have been found at Commentry, a French locality which may be expected to increase the number largely, and about twenty undescribed species are known to me from the American Carboniferous rocks.
The two tribes or sub-families differ in the structure of the mediastinal vein; in one type (Blattinariæ) the branches part from the main stem as in the other veins, at varying distances along its course (see the figure of Etoblattina); in the other (Mylacridæ) they spread like unequal rays of a fan from the very base of the wing (see the figure of Mylacris). What is curious is that the latter type has been found only in the New World, while the former is common to Europe and America. The latter appears to be the more archaic type, since it is probable that the primeval Insect wing was broad at the base, as is the general rule in palæozoic wings, and had the veins somewhat symmetrically disposed on either side of a middle line; in this case the mediastinal and anal areas would be somewhat similar and more or less triangular in form, and the space they occupied would be most readily filled by radiating veins; such a condition of things, which we find in the Mylacridæ, would naturally precede one in which the mediastinal vein, to strengthen the part of the wing most liable to strain, should, as in the Blattinariæ, follow the basal curve of the costal margin, and throw its branches off at intervals toward the border, much after the fashion of the mediastinal vein.
