Fig. 391. The first intermediate co-ordinate network, with its corresponding inscribed pelvis.
birds from the North American Cretaceous formations—a bird shewing some resemblance to the modern terns. The pelvis of Archaeopteryx is taken as our type, and referred accordingly to {759} Cartesian co-ordinates (Fig. [388]); while the corresponding coordinates of the very different pelvis of Apatornis are represented in Fig. [389]. In Fig. [390] the outlines of these two co-ordinate systems are superposed upon one another, and those of three intermediate and equidistant co-ordinate systems are interpolated between them. From each of these latter systems, so determined by direct interpolation, a complete co-ordinate diagram is drawn, and the corresponding outline of a pelvis is found from each of
Fig. 392. The second and third intermediate co-ordinate networks, with their corresponding inscribed pelves.
these systems of co-ordinates, as in Figs. [391], 392. Finally, in Fig. [393] the complete series is represented, beginning with the known pelvis of Archaeopteryx, and leading up by our three intermediate hypothetical types to the known pelvis of Apatornis.
Among mammalian skulls I will take two illustrations only, one drawn from a comparison of the human skull with that of the higher apes, and another from the group of Perissodactyle {760} Ungulates, the group which includes the rhinoceros, the tapir, and the horse.
Fig. 393. The pelves of Archaeopteryx and of Apatornis, with three transitional types interpolated between them.