To return now to the Triassic deposits of Colorado, we recognize here an assemblage of forms of a strictly intermediate character. Here are _Palæoblattariæ_ and _Neoblattariæ_, side by side. The larger proportion are _Palæoblattariæ_, but all of them are specifically, and most of them generically, distinct from palæozoic species, and all rank high among _Blattinariæ_; still further, the species are all of moderate size, their general average being but little above that of mesozoic Cockroaches, and only a little more than half that of palæozoic types. The _Neoblattariæ_ of this Triassic deposit are still smaller, being actually smaller than the average mesozoic Cockroach, and one or two of them, of the genus _Neorthroblattina_ (see figure of _N. Lakesii_), have marked affinity to one of the genera of Palæoblattariæ (_Poroblattina_) peculiar to the same beds, differing mainly in the union or separation of the mediastinal and scapular veins; while others, as _Scutinoblattina_, have a _Phoraspis_-like aspect and density of membrane. This novel assemblage of species bridges over the distinctions between the _Palæoblattariæ_ and _Neoblattariæ_. We find, first, forms in which the front wings are diaphanous, with distinct mediastinal and scapular veins, and the anal veinlets run to the border of the wing (_Spiloblattina_, _Poroblattina_); next, those having a little opacity of the front wings, with blended mediastinal and scapular, and the anal veins as before (some species of _Neorthroblattina_); then those with still greater opacity, with the same structural features (other species of _Neorthroblattina_); next, those having a coriaceous or leathery structure, blended mediastinal and scapular, and anal veins falling on the inner margin (some species of _Scutinoblattina_); and, finally, similarly thickened wings with blended mediastinal and scapular, and anal veins impinging on the anal furrow (other species of _Scutinoblattina_). It is not alone, however, by the union of the mediastinal and scapular stems that the reduction of the veins in the wings of later Cockroaches has come about; for in many mesozoic types the externomedian vein is blended with one of its neighbours, while in others not only are the mediastinal and scapular united, but at the same time the externomedian and internomedian.