404 THE ANATOMY OF VEETEBEATED ANIMALS. 



ankylosed tympanic and periotic bones are contained. 

 The ex- and supra-occipitals, together with the inter- 

 parietals, form the whole back wall and middle of the roof 

 of the cranium, separating the parietals completely, and 

 the frontals largely, and reaching the nasal bones. 



The basi-sphenoid is ankylosed with the small and almost 

 horizontal alisphenoids, and there are no sphenoidal ptery- 

 goid 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 com- 

 pletely ankylosed together, where they form the front wall 

 of the brain-case. Posteriorly and above, they divei'ge to 

 receive the interparietal. The supra-orbital processes are 

 extremely large, and are directed forwards and outwards, 

 not backwards and outwards, 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 upwards, and are convex towards 

 one another and to the middle line. Corresponding grooves 

 exist on the under side of the expanded proximal ends of 

 the maxillaries; and when these are in their naturalpositions, 

 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 skuU and with the Eustachian tiibes. 



The narrow premaxillae are ankylosed with the inner 

 margins of the maxillae, and contribute only a very small 

 portion of the alveolar margin of the upper jaw. The 

 alveoli ai-e not completely separated from one another. 

 The pterygoid bones do not iinite in the palate. They have 

 a peculiar excavated 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 apertui-es are left quite 



