and but very little elevated. The horizontal portion is very massive and rounded to support the great molar teeth; it unites with its fellow in front in a narrow, prolonged, spoutlike symphysis.

The stylohyals are forked at their upper extremity, the posterior process being greatly developed. They taper below to a point which is connected by a long ligament with the basihyal. The thyrohyals are long, compressed, and ankylosed to the basihyal.1

1 See A. H. Garrod, Proc. Zool. Soc. 1875, p. 365.

CHAPTER XII.

THE SKULL IN THE CETACEA AND THE SIRENIA.

Order CETACEA.-The animals of this order exhibit some remarkable modifications in the characters of the skull.

I will first select for description that of a young example of one of the Odontoceti or Toothed Whales, the common round-headed Dolphin or Globicephalus of our coasts. Fig. 65 represents a vertical median section of this skull. It will be seen that the cerebral cavity is of a very unusual shape, being short and broad, but extremely high and contracted above—in fact, somewhat in the form of a truncated cone, with rounded edges. The bones of the basicranial axis are curved upwards at each extremity. They consist of the ankylosed basioccipital (BO) and basisphenoid (BS) separated by a vertical fissure from the presphenoid (PS) and mesethmoid (ME), which are also ankylosed, though their original line of separation can still be traced. The pituitary fossa scarcely forms a distinct concavity, and the clinoid processes are almost obsolete. The mesethmoid is very large, and consists of (1) a high and broad vertical plate, which closes in the vacuity between the frontals in the anterior part of the cerebral cavity, and corresponds to

P

the cribriform plate of the ordinary Mammal, though with but few and small perforations; (2) an anterior rod-like, somewhat compressed, pointed prolongation from the lower part of this plate, which extends forwards in the groove of the vomer (Vo) almost to the extremity of the rostrum, and which in great part remains permanently cartilaginous. This corresponds with the septal cartilage of the nose of other Mammals, although owing to the altered position of the nares it has here little relation with these passages.

The cranial cavity is formed chiefly of the cerebral fossa, the cerebellar fossa being relatively small, and the olfactory fossa entirely wanting.

The optic nerve passes out through a deep notch, sometimes a foramen, in the hinder border of the orbitosphenoid. The alisphenoid is not perforated, the foramen rotundum being confluent with the large sphenoidal fissure, and the foramen ovale with a large infundibuliform opening between the alisphenoid, parietal, exoccipital, basioccipital, and basisphenoid, in the bottom of which is seen the inner surface of the periotic (Per), which in the Cetacea makes no projection into the cerebral cavity. The anterior part of this opening corresponds to the foramen lacerum medium with the foramen ovale, the hinder part to the foramen lacerum posterius.1 The squamosal (Sq) appears in the outer boundary for a very small space, between the parietal and the exoccipital. The condylar foramen pierces the exoccipital, near its anterior edge. The large or nearly circular carotid canal has a peculiar position, passing through the basisphenoid, near its middle, in a direction from below upwards, forwards, and inwards.

1 In the adult of the same species, the foramen ovale is separated from the large opening common to the seventh and eighth pair of nerves by a strong bony partition formed by the ossified tentorium cerebelli.

The bones forming the walls of the cranial cavity are disposed in a very remarkable manner. The occipital surface is of great size, and slopes upwards and forwards. The foramen magnum is large, and looks directly backwards; its

FIG. 65. A section of the skull of a young Dolphin (Globicephalus melas), . PMx premaxilla; Mx maxilla; ME ossified portion of the mesethmoid; an anterior nares; Na nasal; IP interparietal; Fr frontal; Pa parietal; SO supraoccipital; Exo exoccipital; BO basioccipital; Sq squamosal; Per periotic; AS alisphenoid; PS presphenoid; Pt pterygoid; pn posterior nares; Pl palatine; Vo vomer: s symphysis of mandible; id inferior dental canal; cp coronoid process; cd condyle; a angle; sh stylohyal; bh basihyal; th thyrohyal.

lower lateral margins are bounded by large oval condyles, which meet in the middle line below, and are formed by the exoccipitals, with a small portion of the basioccipital. Above the foramen, the immense supraoccipital (SO), with which an interparietal (IP) is ankylosed, extends forwards.

beyond the vertex, to be wedged in between the frontals, completely excluding the parietals from the upper region of the cranium. These latter (Pa) form the greater part of the sides of the narrow high temporal fossæ, and are ankylosed with the supraoccipital above, although the different elements of the occipital are still distinct. The frontals (Fr) are broad from side to side, being prolonged outwards into the arched supraorbital plates, but are almost entirely covered by lamelliform extensions of the maxillæ, which leave but a thin strip of the frontals visible on the external surface of the cranium. The temporal fossa is bounded below and in front by a stout postorbital process of the frontal, very nearly meeting the broad zygomatic process of the squamosal. The orbit is elongated from before backwards; at its anterior extremity is a rounded antorbital prominence, formed by the junction of the maxillæ, frontal and malar; below, it is bounded by a long and very slender styliform zygomatic process of the malar, which arises from near the anterior and inner angle of the body of the bone, and passes backwards, slightly curved downwards, to articulate with the extremity of the zygomatic process of the squamosal. There is no distinct lacrymal bone, or canal.

The special modification of the bones of the face has relation chiefly to the peculiar position of the nasal passages, which instead of passing forwards above the roof of the mouth to the anterior extremity of the face, are directed upwards and somewhat backwards towards the vertex of the cranium; the external narial orifices being situated quite on the top of the head, the part which first appears above the surface of the water when the animal rises for the purpose of respiration. The whole nasal cavities are small, and they are (as far as concerns their bony walls) simple canals, entirely destitute of turbinals. Though their direction is in the main