frontals. The orbito-sphenoid and alisphenoid vary in size. The squamosal is large, and is firmly fixed to the side of the skull, forming part of the wall of the cranial cavity. The periotic, usually anchylosed into one bone with the bullate tympanic, sometimes enters largely into the wall of the cranium, sometimes is almost altogether excluded therefrom by the parietal, alisphenoid, and other adjacent bones, which send prolongations over it.

The maxillary apparatus is greatly elongated, so as to form

Fig. 107.-Upper, under, and side views of the skull of a foetal Whalebone Whale (Balana australis). The jugal bones are absent. In the under view the palatine bone is accidentally marked Pt instead of Pl.

a kind of beak.

The premaxillæ enter into the upper and inner part of the whole length of this maxillary beak, but contribute little or nothing to its palatine surface and lateral boundaries, which are formed mainly by the maxillæ. The latter bones are always prolonged over, or in front of, the supra-orbital processes of the frontals.

The imperforate lachrymal is small, and sometimes coalesces with the jugal.

The nasal bones are always short, sometimes rudimentary; and the palatine bones are so disposed that the posterior nares are situated almost vertically under the anterior nares.

The squamosal bones are produced outwards, and the processes thus formed approach, or come into contact with, the posterior part of the supra-orbital processes of the frontals, which they separate from the jugal. Inferiorly, these processes support the glenoidal facets for the condyle of the lower jaw.

The sides of the broad basi-occipital are always prolonged downwards into free plates, which are concave outwards. These plates join the pterygoids in front, and the ex-occipitals behind, and so constitute the inner and posterior walls of an auditory chamber, the anterior and outer boundaries of which are furnished by the alisphenoid and the squamosal. In this chamber the tympano-periotic is lodged, sometimes quite loosely, at others fixed firmly in by interlocking sutures.

In the Balanoidea, or "Whalebone Whales," the symmetry of the skull is undisturbed, though there may be a slight inequality of the maxillæ. The skull of the foetal Balaena australis, represented in Fig. 107, is perfectly symmetrical. Each lateral edge of the broad and flat basi-occipital is prolonged downwards and outwards into a broad process, concave outwards and convex inwards, the inferior edge of which is free, while the hinder edge unites with the ex-occipital, and the front edge with the pterygoid, to form the inner wall of the funnel-shaped chamber which lodges the tympano-periotic bone.

In front, this chamber is bounded by the pterygoid and the squamosal, and between and above them, for a small space, by the alisphenoid; behind, it is constituted almost entirely by the ex-occipital, while, externally and above, it is bounded and roofed

in by the squamosal. Between these bones there is left, at the apex of the chamber, a considerable irregular aperture, which communicates with the cranial cavity.

The anterior and outer part of the under-surface of the squamosal is produced downwards into a great trihedral pillar, the obliquely truncated inferior face of which bears the articular surface for the mandible (Gl, Fig. 107). Behind this the squamosal presents a comparatively low wedge-shaped ridge (a, Fig. 108), between which and the "trihedral pillar" is a groove; while behind it, or between it and the ex-occipital, there is a deeper and wider transverse channel.

Fig. 108.

The periotic bone is irregularly triangular; the apex of the triangle, turned inwards and forwards, is thick and rounded, the a anterior, posterior, and outer edges being thinner and more or less Opo irregular. The upper smooth and concavo-convex surface of the Fig. 108.-Enlarged view of the chamber periotic adjusts itself to the underbone of the fatal Balana australis. surface of the squamosal, where it a, the "wedge-shaped ridge" of the forms the roof of the funnel-shaped squamosal; C, the aperture which leads into the interior of the skull; cavity. The apex of the periotic, f.o, fenestra rotunda. however, projects beyond this, and incompletely divides the irregular aperture above mentioned (b, Fig. 108) into an anterior division, which corresponds with the foramen ovale and foramen lacerum medium, and a posterior which answers to a foramen lacerum posterius.

The under-surface of the periotic, much more irregular, is divisible into three regions: an outer anterior; an outer posterior; an internal. The first and second are separated by a deep triangular notch in the outer margin of the bone, into which the inner end of the wedge-shaped ridge of the squamosal is received. The first, broad and short (Pr0), presents a rough surface in front, with which the tympanic articulates, and eventually anchyloses; and behind, a concave surface, which, entering into the roof of the tympanic cavity, answers to the

tegmen tympani. The second, narrower, elongated, and prismatic, fits into the transverse channel behind the wedge-shaped process (a). It corresponds with the pars mastoidea, and its rough outer extremity appears on the exterior of the skull, between the squamosal and ex-occipital.

The internal division, convex and rounded below, is formed by the pro-otic and opisthotic, and presents a large promontory with the fenestra rotunda (f.o) on its posterior surface, while the fenestra ovalis and Fallopian canal are visible upon its exterior. The tympanic bone (Fig. 109, Ty) is large, and scroll-like in form, very thick internally and below, and thin above and externally, where it presents the aperture of the external auditory meatus. It is by this thin upper and outer edge only, that it eventually anchyloses with the tegmen tympani and pars mastoidea, and hence, as its substance is very dense and brittle, readily breaks off.

In the adult Whale the tegmen tympani and pars mastoidea

T

become greatly elongated and very rugged, the tympanic also acquiring a very large size (Fig. 109).

The vomer is a very long and large bone, deeply grooved above for the ethmoidal cartilage, which extends downwards and forwards between the premaxillæ and the maxilla to near the anterior end of the snout. Its expanded upper and posterior end unites with the basi-sphenoid in the middle line, and with the pterygoid laterally. In front of the basi-sphenoid it embraces, not a distinct presphenoid (as in Pterobalæna, according to Eschricht), but the inferior surfaces of the orbitosphenoids, which are very thick; and, being applied together by their flat median faces, apparently replace the proper presphenoid.

Both these bones and the alisphenoids are small, and almost confined to the base of the skull.

The supra-occipital and inter-parietal are united together, and completely overlap and hide the parietals in the roof of the skull. The separate frontals only enter into the anterior wall of the skull, and between them and the orbito-sphenoids an oval aperture is left, doubtless diminished in the recent state by the ethmoidal cartilage. Laterally, the frontals are prolonged outwards and backwards into two great supra-orbital processes, which nearly meet the zygomatic processes of the squamosal. The short jugal bones, absent in the specimen figured, extend in the Balenoidea from the zygomatic process to the anterior and external angles of the supra-orbital prolongations, and are distinct from the lachrymals.

The pterygoids are completely separated by the palatines (Fig. 107). In front of the latter the maxillæ almost wholly exclude the premaxillaries from the palate, while they send great processes obliquely outwards and backwards, in front of the supra-orbital prolongations of the frontal. The long premaxillæ, on the other hand, pass upwards and backwards on each side of the elongated and symmetrical nasals to meet the frontals, and exclude the maxillæ altogether from the anterior nares.

The rami of the lower jaw are very narrow, and so much arched outwards as to be able to enclose the baleen plates attached to the upper jaw when the mouth is shut.