408 Bulletin American Museum of Natural History. [Vol. XXVII, 



rium as might be expected in a land animal, the olfactory lobes are more 

 developed than in Eosiren. ... It is interesting to note further that in the 

 Middle Eocene forms of both these orders [Proboscidea and Sirenia] the brain 

 is relatively large for mammals of that early date." 



The scapula of Mceritherium. (Andrews, op. cif., pi. xi, fig. 5) resembles 

 that of Eosiren (pi. xx, fig. 3) in the backward prolongation of the top of 

 the blade, obliquely placed spine and large prespinous fossa (aquatic adap- 

 tations). In both genera the humeri although of very different ])roportions 

 present points of similarity in the form of the distal trochlea and inner con- 

 dyle; in both the deltoid ridge continues down to a point near the internal 

 condyle, and both lack the entepicondylar foramen. The pelvis of Eotherium 

 as noted by Andrews (op. cit., p. 214) is in many respects intermediate in 

 character between those of the Sirenian Halitherium and the supposed 

 Proboscidean Mceritherium. The ilium of Mwritherimn is not expanded 

 as in Proboscidea but trihedral, with an ileo pectineal tubercle at its base, 

 the ischial tuberosity is very broad, the thyroid fenestra relatively small and 

 the posterior })art of the pubic bar slender; these characters, with some 

 changes in proportion, are also seen in Eotherium; the acetabulum of which 

 is well developed, proving that in this most primitive Sirenian the hind limb 

 was still functional. The skull of Eotherium (Andrews, op. cit., p. 205) while 

 already much modified in the Sirenian direction tends on the whole to con- 

 firm the view that the Sirenia, Proboscidea and Hyracoidea are divergent 

 offshoots from a single stock. 



Among the characters which the skulls of Eotherium. and Mceritherium 

 exhibit in common some may be conceived to have developed more or less 

 independently, such as: (1) The retraction of the nasals (very early reaching 

 an extreme in the Sirenia) : (2) the enlargement of one pair of incisors (i^ in 

 Eotherium, r in Mceritherium and the Proboscidea) ; (3) the prolongation of 

 the palate and straightness of the tooth row; (4) the development of bilopho- 

 dont molars; (5) the broatlening of the coronoid, the anterior edge of which 

 is external to the ])osterior molars. The skulls however show further 

 resemblances which are less obviously adaptive, e. g., (1) the extremely 

 forward position of the orbits, a point emphasized by Osborn (2) the 

 resemblances in the occipital aspect of the skull ; the mastoid being entirely 

 concealed in Mceritherium and nearly so in Eotherium; while the upper 

 posterior angle of the squamosal in " Eotherium is shut off from contact with 

 the supra-occipital by a narrow posterior prolongation of the parietal which 

 is wedged in between the two bones somewhat as in Mceritherium" (Andrews, 

 op. cit., p. 205). (3) The base of the cranium is rather similar in the two 

 forms. Eotherium, however, retains a condylar foramen, unlike the Pro- 

 boscidea and Embrithopoda, in which the condylar foramen is generally 

 confluent with the foramen lacerum posterius. 



