| Case. | Comp. | Tablet. | |
| J | c | 8 | 1 |
Back of the Cranium.
[Pl. 11.]
This fossil is an inch high, rather wider, and half an inch long. It well shows the bones at the back of the skull, the basi-cranial bones, and the bones posterior to the frontals, which roof in the Cranium. There are in it striking resemblances to the back of the skull of some Natatores, as the Grannet and Cormorant, and of some Grallatores as the Heron, and Gallinaceous birds as the Cock.
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.
The occipital bones make with those at the base of the skull an angle of about 145° or 150°. In outline they are a transverse diamond shape. The mastoid portion is not to be distinguished from the other bones, but appear to terminate the sides of the strong occipital crest, which by posterior compression of the squamosals and parietals, becomes very strong, and makes the backward boundary of the temporal foss. This crest is in the same plane with the anterior border of the basi-temporals.
The parietals meet above in a slight ridge. They are two rectangular bones twice as wide as long, forming a semicircular roof for the brain, which looks outward and a little backward. Anteriorly these bones unite with the frontals in a slightly flexuous transverse line; and inferiorly they are connected with the periotic, the squamosal, and perhaps with the anterior point of the alisphenoid: they do not descend to the plane of the articulations of the free quadrate bones. The surface is smooth, and on the upper part flat, but concave below from side to side.
Below these parietals are the squamosals and alisphenoids, but the suture between them is not seen. They are in form a trapezium where the short side is anterior, and the lower third is folded inward so as to be confluent with the anterior part of the sphenoid. The fold forms a ridge, which I suppose may run obliquely over the alisphenoid. The unfolded squamosal part is a flat and smooth oblong, with parallel sides, the bones are in parallel planes and nearly perpendicular to the base of the skull. Where the alisphenoid joins the sphenoid, there is a considerable concavity, above which is a small circular impression. These strips approximate inferiorly, so that the width of the skull there is rather more than half what it is at their outer margins. They shut off the pituitary body in front of them, and appear to form part of the wall for the orbit of the eye.—The slightly convex, lateral, squamosal parts above the fold continue the circular transverse outline of which the parietals are the upper half. They extend anterior to the parietals, and on the inside give attachment to the frontals. Like the parietals, they make a sharp bend outward at their hinder border, and form the lateral terminations of the occipital ridge, which is the widest part of this fossil.