DE. J. MUKIE ON THE FOEM AND STEUCTUEE OF THE MANATEE. 141 



cerebral lobes. An ethmoidal elevation or ridge of bone is well marked ; outside it 

 (partly seen in fig. 36) is the broad depression which lodges the olfactory bulb (1) ; the 

 optic foramen (2) at its posterior border is barely distinguishable. A narrow elliptical 

 slit (o) pierces the outer wing of the arched transverse fibrous septa, and transmits the 

 fifth nerve. In the posterior fossa, behind the septa and partly to the inner side of the 

 slit spoken of, there is a broad hollow (t), wherein the temporal lobe of the brain lies. 

 About the centre of the cranial floor and within the area of the somewhat triangular 

 inner horn of the transverse fibrous septa from before backwards, are as follows : — a 

 tiny elevation representative of the lesser wing of the sphenoid ; outside it a perforation 

 in the membrane corresponding to the sphenoidal fissure (3), transmitting the third and 

 fourth nerves &c. ; behind that a shallow pituitaiy fossa (jj) ; and lastly, posteriorly, an 

 oblique groove and membranous perforation for the carotid arteiy (ca). 



About the middle of the posterior fossa, in the valley of the nodular petrous eleva- 

 tion, is the large internal auditory foramen (7). Eearwards of this, at the occipital, is a 

 wide groove running outwards and backwards, the membranous covering of which 

 hides the jugular portion (J) of the foramen lacerum posterius. Outwards from this, in 

 an angular corner of our section, are the great vascular plexus (Px) and vein which fill 

 the deep lateral sinus. The spinal vascular plexus (Px*), which communicates with 

 this, occupies a considerable portion of the foramen magnum. 



On removal of the dura mater (as the opposite half of the figure shows), the bones of 

 the cranial basis display material distinction from the fossa as covered by the membrane. 

 They agree in most respects with the allied form Halicore, to some extent with the 

 young of Elephas, and certainly resemble these more than they do Cetaceans. 



The frontal, fenestrated plate, and stout crista galli of the ethmoid foim the anterior 

 perpendicular cranial wall, partly assisted below by the fair-sized orbito-sphenoid {o.s). 

 The cribriform ethmoidal lamella is oval, fully half an inch in vertical depth, and freely 

 perforated. A groove leading to the small optic foramen (2*) runs obliquely outwards; 

 and immediately external to this is a large irregular ovoid perforation, separating the 

 orbital from the alisphenoid. This interspace, one inch long by half an inch broad in 

 our female specimen, is partly freely open and partly blocked up by a semiglobular 

 projection of the dental portion of the maxillary bone {Mx), which in this case con- 

 tained the germs of molars. 



The alisphenoid plate {As), ankylosed with the basisphenoid (-Bs), constitute the 

 mid floor ; the former constitutes a part of the lateral cranial wall, being wedged 

 in between the squamosal parietal and tip of the orbito-sphenoid. The suture betwixt 

 the basioccipital and basisphenoid is distinct, that between the latter and orbito-sphenoid 

 less so. A pair of aborted nipple-like leaflets of bone appear to represent the anterior 

 clinoid processes ; and behind these are a few foramina. 



Backwards from the basisphenoid is the flat bar of the basioccipital (Bo), which, 

 forking outwards with the inner spur of the exoccipital {Eo), ciixuit the lower margin 



