It is gradually becoming recognised that the class Reptilia is not a monophyletic group of diverging forms sprung from a common stem, like the class Aves or the class Mammalia; but is an assemblage containing, on the one hand, the ancestors of the Mammalia, and, on the other hand, the ancestors of the Birds, together with the early Amphibian-like Amniotes, which became adapted to a terrestrial mode of life. In fact, the Reptilia represent not a class but a grade of structure. This group includes a main stem leading from the Stegocephalian type to a central point of divergence of two main branches, one giving rise to the Birds, the other to the Mammals (as shown in the diagram, fig. 1). In addition, there are, of course, many abortive side twigs. Some day, no doubt, when the exact relationship of the various living and extinct reptiles has been more accurately determined, it will be necessary to split up the artificial group Reptilia, assigning some to the Mammalia and some to the Aves; but for the present we may be content with keeping the class Eeptilia, always remembering that it is a grade of ill-defined limits. The modern views of the phylogenetic relationships of the various orders of Eeptilia may be said to have arisen chiefly from the work of Cope and Baur, following on the conclusions of Huxley and other earlier authors. To the ingenuity of Cope we owe the valuable suggestion that the starting-point of the divergence between the Amphibia and the Eeptilia was determined by the structure of the vertebral column—the vertebral body being mainly derived from the hypocentrum in the former and from the pleurocentrum in the Amniota. Being thus provided with a means of distinguishing the early reptiles from their Amphibian relatives, the next step is to seek for characters enabling us to trace out the diverging lines among the Eeptilia themselves. Here again we are indebted to Cope (13), but more especially to Baur (1, 2) for pointing out the importance of the roofing of the skull in classification. Whereas the earliest and most primitive reptiles have, like their Amphibian ancestors, a roofing complete over the temporal region, this becomes pierced in others by one or two foramina. Thus are left one or two longitudinal temporal arches. The formation of the foramina or fossæ is generally accompanied by a reduction in the number of bones covering the hinder region of the skull. It is not my intention to enter into a detailed account of these points in this paper; they have been discussed by many authors, and are well understood. It will be sufficient for our present purpose to point out how profoundly the modern classification of the Amniota has been affected by their recognition.
I - Biology and morphology of the immature stages of Mycetophilidae (
Diptera, Nematocera
)