INTRODUCTION. xxi 



(p. 204) at some length. Eosiren, though primitive, is already in all respects a typical 

 Sirenian, almost the only important characters distinguishing it from the later members 

 of the order being : (1) the presence of traces of the second and third pairs of incisors 

 and of the canines ; and (2) the rather less degree ol reduction in the pelvis (see fig. 68 B, 

 p. 214), v?hich still possesses a well-defined acetabulum. In the case of the incisors 

 and canines reduction is already far advanced and they have also been thrust out on 

 to the side of the snout, possibly by the development of the horny plate, which 

 most likely already replaced them functionally. In Eotherium, from the lower beds 

 of the Mokattam Hills, the incisors and canines are larger and occupy their normal 

 position on the edge of the jaw, and the pelvis has a completely closed obturator 

 foramen and a large and well-defined acetabulum, showing that probably the femur 

 was still large and perhaps to some degree functional. In these points Eotherium 

 approaches a normal land-mammal, but in other respects, so far as known, is 

 essentially a Sirenian and its actual terrestrial ancestor must be sought in earlier 

 deposits. One of the most striking Sirenian characters of the skull in both this genus 

 and Eosiren is the deflection of the snout, a peculiarity evidently of great value to a 

 short-necked, long-bodied, aquatic animal feeding on plants growing at the bottom 

 of the water in which it lived ; in the most primitive type of Sirenian, the imperfectly 

 known Prorastomus described by Owen (Quart. Journ. Geol. Soc. vol. xi. 1855, p. 541) 

 from the Eocene beds of Jamaica, this character has not yet been acquired. 



The question of the origin of theSirenia is of great interest, and there seems to be a 

 considerable amount of evidence in favour of the view first put forward by de Blainville, 

 that they are intimately related to the Proboscidea. In the first place, the occurrence 

 of the most primitive Sirenians with Avhich we are acquainted in the same region as the 

 most generalised Proboscidean Moeritherium is in favour of such a view, and this is 

 further supported by the similarity of the brain-structure and, to some extent, of the 

 pelvis in the earliest-known members of the two groups (see pp. 202 & 214). Moreover, 

 in the anatomy of the soft parts of the recent forms there are a number of remarkable 

 points of resemblance. Among these common characters may be noted the possession of: 

 (1) pectoral mammae, (2) abdominal testes, (3) a bifid apex of the heart, (4) bilophodont 

 molars with a tendency to the formation of an additional lobe from the posterior part 

 of the cingulum. The peculiar mode of displacement of the teeth from behind 

 forwards in some members of both groups may perhaps indicate a relationship, 

 although in the case of the Sirenia the replacement takes place by means of a 

 succession of similar molars, while in the Proboscidea the molars remain the same 



d 



