_The base of the skull._ The bones here indicated are the basi-occipital, basi-temporal, and basi-sphenoid. The former two have come away as from an articular joint, and are wanting. The basi-occipital does not enter into the floor of the cranial cavity, and only rims the foramen magnum. But its basi-temporal expansion rests beneath the posterior part of the basi-sphenoid forming the base of the skull; its long convex anterior end fits into the concave groove at the back of the anterior part of the sphenoid. The squamous basi-temporal bone appears in this species to have been as long as the foramen magnum is wide, and to have been relatively thicker than in the other form already described.
The _basi-sphenoid_ is a thin expanded bone forming the floor for the cerebellum, and terminating anteriorly in a triangular mass, while the slightly convex part behind, covered with the basi-temporals, is nearly square. It enters into the foramen magnum, forming its lower part; and is confluent with the ex-occipitals behind, with the periotic, alisphenoid and perhaps with the squamosal at the side; and as in birds all these sutures are obliterated. This is probably the only instance in the Animal Kingdom in which the basi-sphenoid takes so important and singular a share in the functions of the basi-occipital bone. The anterior part of the basi-sphenoid projects below the posterior part, is nearly flat on the basal surface, and forms an equilateral triangle with the apex in front and base behind. In the middle of the triangular bone is a slight longitudinal ridge, and behind the middle of each outer side a rather large foramen which appears to be the inferior opening for the carotid artery. The triangular part is hollow and as long as the quadrate portion. The lateral parts of this anterior bone are nearly flat. They converge upwards and are rounded in front to form the boundary of the pituitary fossa, and do not appear to have terminated in a spine. Above are the alisphenoids.
_The upper part of the skull_ is divided into two segments by a strong straight transverse ridge, which leaves the occipital bones behind, and the parietal &c. in front.
The occipital bones anchylosed together are about two-thirds the width of the foramen magnum, and of the parietal bones, with which latter the supra-occipital makes an angle of 45°. The surface is irregular, and especially is marked by a deep concavity just above each ex-occipital. The supra-occipital projects slightly over the plane of the foramen magnum, to which the strong ridge bounding the segment in front is parallel. The great foramen is nearly round, being slightly compressed at the upper part of the sides: it measures 3/8 of an inch high and is nearly as wide.