OF AN EOCENE SIRENIAN MAMMAL. 



101 



tinct Artiodactyle ; and, as in the latter comparison, the defect of 

 cerebral development in the Eocene prototype is more in regard to 

 breadth and depth than to length of brain. 



The supporting floor of the macromyelon is bounded anteriorly 

 in Manatus by a shallow transverse linear depression, about one 

 line behind the " sella ; " its length from the lower margin of the 

 foramen magnum is 1 inch 8 lines. 



A transverse ridge r (PI. III. fig. 3), crossing behind the swelling 

 representing the infundibulum (p) in the cast of the cranial cavity of 

 Eotherium, has filled a similar but deeper transverse groove, and 

 marks the anterior limit of the macromyelon in the Eocene Sirenian. 

 From this mark to the hind fracture, which appears to have taken 

 place just beyond the foramen magnum, the length of the macro- 

 myelon may be taken at 1 inch 9 lines. 



The median mass b (PL III. fig. 3) indicates the enlarged anterior 

 myelonal columns ; they are defined by a shallow depression from 

 the lateral or " restiform " columns (#). The " pons Varolii " is 

 indicated at a. A depression (c, fig. 2) on each side the posterior or 

 dorsal surface of the macromyelon indicates the limit between the 

 posterior myelonal columns and the post-restiform tracts. The section 

 of the macromyelon at the foramen magnum is shown in fig. 4. It 

 is broader than it is deep, but rather less so than in Manatus. The 

 cast of the cranial cavity in this Sirenian, showing the brain as 

 covered by its membranes (fig. 5), indicates the prominence of the 

 pons Varolii as feebly as in the fossil. The size of the cerebellum is 

 marked in a greater degree in such casts than is that of the pons. 

 The median series of folds, or ' upper vermiform process,' makes a 

 slight prominence in the fossil (fig. 2, v) ; but the relative size of the 

 cerebellum to the cerebrum is characteristically small, as in the 

 existing Sirenia (fig. 5, v). The breadth of the epencephalic chamber, 

 as represented by the cast, is 2 inches : there is a slight contraction 

 between this and the prosencephalic chamber, which soon swells out 

 to a breadth of 2 inches 4 lines j this gives the extreme breadth 

 of the cerebrum in Eotherium. The length of the cerebrum is 

 2 inches 6 lines ; the breadth of the anterior lobes is 2 inches. 

 The prominence anterior to the Sylvian fissure (figs. 1 & 5, 5), 

 answering to that marked n in pi. xxv. fig. 32, Zool. Trans, vol. viii., 

 is recognizable in the fossil. The upper and lateral surfaces of the 

 casts (figs. 1 & 2) indicate the coarse fibrous character of the dura 

 mater, such as this membrane shows in recent Sirenians. The 

 pedicle of the pituitary body is marked by a small prominence 

 (fig. 3, p) anterior to that indicating the infundibulum. On each 

 side of this part the optic nerves (0 0) are indicated, converging as 

 they advance. External to the optic nerves are the large trigeminal 

 ones (tr), of which about an inch and a quarter remain, with a 

 thickness, as covered by the dura mater, of 4| lines. The rhinen- 

 cephala (olfactory bulbs) are of moderate size (figs. 1-3, k). The 

 depression dividing the hemispheres above is rather wide along the 

 mid line ; a narrow ridge of the cast along the fore half of this line 

 indicates a depression for the insertion of the falx (fig. 2, /), 



