It is well known that the main functional components of the skull in the Whalebone Whales (Mystacoceti) attain an ultimate size, which is out of normal proportion to the rest of the skeleton as compared to the proportions existing in land-mammals. As generally supposed the aquatic mode of life has greatly influenced the shape of the skull. Therefore, it is to be expected that the various species, which all have the same mode of life, show a certain amount of convergence. However, not all bones have become enlarged to the same degree; e.g., the alisphenoid and the orbitosphenoid have remained relatively small. Apparently these bones of the orbit have not been influenced by the aquatic mode of life to the same extent as is the case with other elements of the skull. In the present paper some features of the orbit of the Mystacoceti are described; the variability and the systematic importance of these peculiarities have been examined. In order to indicate the degree of modification undergone by the Whale skull, it is necessary to have a clear idea of the typical arrangement of the elements bordering the orbit in the skull of a primitive mammal (fig. 1a). This figure shows that the orbit is lined by several elements; of these the alisphenoid and the orbitosphenoid lie in the lateral wall of the skull and they immediately surround the foramen opticum. It also shows that these two bones are contiguous with the frontal, the parietal, and the squamosal. In these respects the skull of juvenile Cetacea shows more resemblance to the primitive mammalian skull than do those of the adults. Comparison of the skulls of juvenile Tursiops tursio (Fabr.) (fig. 1b) and Balaenoptera borealis Lesson (fig. 1C) with those of adult specimens shows that in the adult the shape of some of the orbital bones has under-