DE. J. MURIE ON THE ANATOMY OF THE SEA-LION. 519 



the superficies, especially at its hinder portion. The venous sinuses are prominent, and 

 fit into the remarkably deep grooves already mentioned in the description of the interior 

 of the cranium. There is a considerable thickening of the dura mater as it passes out 

 of the foramen magnum backwards towards the spinal canal. Vascularity also distin- 

 guishes the pia mater, otherwise of an ordinary character. 



3. The Brain. 



a. Its outward aspects and dimensions. — The general characteristic feature of the 

 brain of Otaria, looked at on its upper surface, is its comparative squareness— in this 

 respect differing from the more common ovoid form of mammals generally, as well as 

 from the somewhat circular contour which it assumes in Phoea and particularly in 

 Cetaceans. This quadrilateral configuration is chiefly produced by the abrupt trun- 

 cation of the frontal and occipital lobes respectively, their outer corners being con- 

 siderably angular, or but very moderately rounded. The lateral margins are deeply 

 indented about their middles ; and the fronto-parietal portions are less prominent than 

 the temporo-occipital ones ; nevertheless they, on the whole, still lend something to 

 the general quadrilinear aspect of the entire encephalon. Notwithstanding what has 

 been said, each cerebral hemisphere superiorly presents a reniform outline, the deeply 

 indented Sylvian fissure being equivalent to the hilus, and the straight-edged longi- 

 tudinal fissure to the dorsum. The olfactory bulbs are large, and mesially project 

 considerably forwards. The posterior lobes of the cerebrum are tolerably equal in 

 dimensions : the left may be slightly longer than the right ; but this was not clearly 

 appreciable by measurement, though appearing so to the eye. 



Unlike some of the so-called higher forms of Carnivora, the posterior cerebral lobes 

 all but overlap the cerebellum laterally, as Huxley has recorded is also the case in 

 the allied genus Trichechus. The actual amount of backward projection of the outer 

 cerebellar lobes is little more than OT inch. Mesially, however, the superior vermi- 

 form and superior posterior lobes of the cerebellum are more exposed, have a 

 triangular form IT inch long and 1-3 inch broad, and reach slightly further back than 

 the external lobes. 



The cerebral convolutions are numerous and well developed, giving this upper sur- 

 face quite a sinuous appearance. There is a certain amount of asymmetry between the 

 halves ; but this shall be described hereafter. The brain is highest behind, or at the 

 junction of the occipital with the parietal lobes; and from this it inclines downwards 

 and forwards, as also more steeply outwards. 



Measured from the anterior extremity of the olfactory lobe backwards in a straight line 

 to the most projecting part of the cerebellum, the total length is 4-6 inches. The dia- 

 meter across the parietal lobes is 3-2 inches. The extreme longitudinal axis of each 

 moiety of the cerebrum is 4 inches. The greatest transverse diameter of the brain, which 

 is about the middle of the occipital lobes, is also about 4 inches. Thus the length of 



4c2 



