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. [Illustration: Fig. 121.--_Mylacris anthracophilum_ Scudd. × 2. Carboniferous, Illinois.] This view of the relative antiquity of the two tribes of _Palæoblattariæ_ is supported by the fact that while in both of them the internomedian branches show a tendency to repeat the general course of the anal nervules, as in the corresponding veins of the costal region, this tendency is lost in modern types; and among those ancient _Blattinariæ_, which are esteemed highest in the series, there is a marked tendency toward a loss of this repetition of the style of branching of the mediastinal and anal offshoots by the scapular and internomedian respectively.