274 MAMMALIA. 



characters ; and the extinct Patagonian Argyrocetus, as already 

 mentioned (p. 272), may well have been one of the ancestral 

 types. 



Unfortunately, Palaeontology has not hitherto furnished any 

 of the links in this process of evolution. It is merely known 

 that small, but typical whalebone whales existed so long ago as 

 the Miocene period both in Europe and North America; the 

 neck in these being only a little longer than in those of the 

 present fauna, the parieto-frontal region of the cranium also 

 somewhat more elongated, and the nasal bones better developed. 

 Cetotherium and Plesiocetus are typical old genera, and a skull 

 apparently of the former is known from the Marine Patagonian 

 Formation of Chubut, Argentine Republic. Fragments of typi- 

 cal modern Balaenidse are not known below the Pliocene. 



ORDER 2. SIRENIA. 



Like the Cetacea, the allies of the existing manatees and 

 dugongs are also known to date back to the early part of the 

 Tertiary period, without as yet affording any clue to the origin 

 and relationships of the order they represent. The early 

 extinct forms already discovered merely differ in exhibiting 

 (i.) a less modified dentition and nasal region, (ii.) distinct 

 disc-shaped epiphyses at the ends of the vertebral centra, 

 and (iii.) well-developed rudiments of the hind limbs. 



The oldest fossil placed here is a natural cast of a brain- 

 cavity (named Eotherium aegyptiacum) from the Middle Eocene 

 Nummulitic Limestone of the Mokattam Hills, near Cairo, 

 where typical Sirenian teeth also occur. Another still more 

 interesting specimen, of great but uncertain antiquity, is an 

 imperfect skull (Prorastomus sirenoides) from Jamaica, dis- 

 playing a very generalized dentition. Halitherium and Miosiren 

 are known by nearly complete skeletons from the Lower 

 Miocene of Germany and Belgium respectively. Several 

 other more fragmentary specimens of Sirenia have also been 

 discovered in the Miocene and Pliocene both of Europe and 

 America ; while the toothless Steller's sea-cow (Rhytina stelleri\ 

 exterminated only in the last century, is represented by nearly 



