344 THE ANATOMY OF VERTEBRATED ANIMALS. 



roof of the cranium, separating tlie parietals completely, and 

 the frontals largely, and reaching the nasal bones. 



The basi-sphenoid is anchylosed with the small and almost 

 horizontal alisphenoids, and there are no sphenoidal pterygoid 

 processes. The parietals are small, and occupy only the under 

 and lateral portions of the brain-case. The frontal bones are 

 very broad and expanded, and are completely anchylosed to- 

 gether, where they form the front wall of the brain-case. Pos- 

 teriorly and above, they diverge to receive the interparietal. 

 The supra-orbital processes are extremely large, and are 

 directed forward and outward, not backward and outward, as 

 in the Whalebone Whales. The greater part of the superior 

 surface of the frontals and of their orbital processes is rough 

 and covered over by the expanded maxillary bones, which 

 allow only a narrow, transverse, smooth, band-like surface, 

 formed by the frontals, to be seen on the upper and anterior 

 region of the skull. The rough surface is marked by two 

 shallow grooves which pass from below upward, and are con- 

 vex toward one another and to the middle line. Correspond- 

 ing grooves exist on the under side of the expanded proximal 

 ends of the maxillaries ; and, when these are in their natural 

 positions, the coadapted grooves form two canals, which are 

 blind in front and above. These, in the natural state, are full 

 of air, and communicate with the air-chambers at the base of 

 the skull and with the Eustachian tubes. 



The narrow premaxillse are anchylosed with the inner 

 margins of the maxillse, and contribute only a very small por- 

 tion of the alveolar margin of the upper jaw. The alveoli are 

 not completely separated from one another. The pterygoid 

 bones do not unite in the palate. They have a peculiar ex- 

 cavated form, and are notched for the passage of the ends of 

 the Eustachian tubes into the nasal passages. These are nearly 

 vertical and are separated by the large and strong vomer. 

 Their superior apertures are left quite uncovered in conse- 

 quence of the small size, tubercular form, and backward posi- 

 tion of the nasal bones. The squamosal is relatively small, but 

 has the characteristically cetacean, large, zygomatic process ; 

 this extends forward nearly to the posterior end of the supra- 

 orbital process, and gives attachment to the slender jugal. 



The periotic bones form a dense osseous mass, which is 

 anchylosed with the no less heavy and thick, scroll-shaped 

 tympanic. Tlie pars mastoidea of the periotic mass fits pretty 

 accurately into a recess of the chamber which has already been 

 described ; and is thus held in position in the dry skull, though 

 it is very easily detached. 



