102 PBOF. OWEN ON EOSSIL EVIDENCES 



The brain in existing Sirenia is characterized by a subquadrate 

 form of the cerebrum, with the angles rounded off, arising from the 

 breadth of the fore part being equal, or nearly so, to the hind part, 

 the cerebrum not gaining, or but slightly gaining, breadth (Manatus), 

 or slightly losing breadth {Halicore) , as it recedes or approaches the 

 cerebellum. The first character is exemplified in the figures of the 

 brain, or its model, of the American Manatee {Manatus americanus, 

 Cuv.), given by Murie *. But the proportion of breadth to length 

 is less in the Eoceue fossil. 



As the figures by Murie represent the brain of the Manatee 

 uncovered by its membranes, I have taken a cast of the interior of 

 the cranial cavity of a full- sized Manatus americanus to compare 

 with the appearances presented by the fossil, and have figured the 

 upper surface in PI. III. fig. 5, and the under surface of the cerebrum 

 and rhinencephalon in fig. 6, of the natural size f . 



In the Manatee {Manatus americanus) the greatest breadth of the 

 parietal roof of the cranial cavity is four tenths of the total length 

 of the skull £ ; in the Wiytina it is a little more than two tenths of 

 that length § ; in Halitherium it is rather less than two tenths. 

 Halitherium and Manatus give the two known extremes in the pro- 

 portions of the breadth of the cranial cavity to the length of that 

 cavity and of the entire skull in Sirenia. The proportions of the 

 brain correspond with those of its bony case. This is exemplified 

 by those of the brain of the Dugong ||, the proportions of the cranial 

 cavity of which are intermediate between those of Manatus and 

 Wiytina. The proportions as to breadth and length of the brain 

 of the extinct Sirenian of the Eocene of Egypt are those of Halithe- 

 rium. From the shape of the brain-case in Felsinoiherium Forrestii, 

 as shown in tav. ii. of Capellini's admirable memoir on that Sirenian %, 

 I infer that the shape of the brain must have been much more 

 nearly that of the cast from Eotherium than is the cast of the brain- 

 case in the recent Manatee. 



The above considerations and comparisons will, I trust, beget the 

 same conviction as with myself, that a Sirenian mammal near akin 

 to Felsinoiherium, Cap., to Halitherium, Kaup, and Metaxitherium, 

 Christol and Gervais, existed in the seas in which the calcareous 

 nummulitic Eocene stone of the Arabian cliffs was deposited ** ; 

 whence the inference may be drawn that such formation was accu- 

 mulated at no great distance from the shore. It is an extreme sup- 



* Trans. Zool. Soc. vol. viii. pi. xxv. figs. 31 & 33. " A cast of the cranial 

 cavity, with its enclosed dura mater, was subsequently made ; and by the help 

 of this cast and the shrunken brain the sketches (pi. xxv.) were drawn " (p. 181). 



t A figure of sucb cast, of half the natural size, is given by Brandt in his 

 ' Symbolae Sirenologicae,' t. ix. fig. 1. 



\ Vrolik, ' Bijdragen tot de natuur. &c, Zennis van den Manatus americanus ' 

 (fol. Amsterdam, 1852), vierde Plaat, fig. 11. 



§ Yon Nordmann, ' Beitrage zur Kenntniss des Knochenbaues der Ehytina 

 StelZeri,' 4to, Helsingfors, 1861. 



|| Brandt, ' Symbols Sirenologicae ' (4to, 1861-8), t. ix. fig. 2, Taf. i. fig. 1. 



^[ Memorie della B,. Accademia delle Scienze dell' Istituto di Bologna, Serie 

 terza, tomo i. p. 605, tav. i.-viii. (1872). 

 ** A Dugong (Halicore tabernaculi, Rupp.) still exists in the Red Sea. 



