EVOLUTION OF THE PROBOSCIDEA. 



113 



to be the first indication of the great relative magnitude they are destined to attain 

 in the later forms. The form of the atlas vertebra in Palceomastodon indicates that 

 the neck was longer than in the later forms. The limb bones, as far as known, differ 

 from those of EJephas in little except size. 



Mceritherium lyonsi, Andrews.* 



The skull, mandible and dentition of Mceritherium are fairly well known and have 

 been briefly described and figured by me. Here, only the points necessary for the 

 purpose in hand need be mentioned. 



The Skull (figs. 14 and 15). — The occipital surface is very wide and its plane nearly 

 vertical. The foramen magnum is large, and the occipital condyles prominent and 

 almost pedunculate. The zygomatic process of the squamosal is very massive and 

 prominent, and the glenoid surface is a broad and ill-defined slightly concave area ; 

 there is no post-glenoid process, but a broad post-tympanic flange is closely applied 

 to the hinder edge of the glenoid surface and encloses the large external auditory 

 meatus precisely in the manner characteristic of the later Proboscidea. The parietals 

 form most of the cranial roof, and like the cranial portion of the squamosals are 

 greatly thickened, so that in one species (M a, ritheri am gracile, Andr. f) there is a distinct 

 inflation of the hinder portion of the sides of the skull, possibly the first indication 

 of the enormous development of diploe found in the later forms. The frontals seem 

 to have been small, and there is no well-defined post-orbital processes. The nasals 

 also were small and the nasal opening very large ; the form of this opening, together 

 with the grooving of the upper surface of the large premaxilke, seem to point to the 

 existence of some kind of small proboscis-like structure. The premaxillse bear the 

 alveoli of three pairs of incisors. The second pair are greatly enlarged, trihedral and 

 downwardly directed tusks, the roots of which run back into the maxillae and probably 

 grew from a persistent pulp. The maxillae bear sockets for a pair of canines and six 

 pairs of cheek-teeth ; there is a prominent zygomatic process which is to a great 

 extent overlapped by the anterior portion of the jugal, which here forms a large part 

 of the zygomatic arch and extends back beneath the zygomatic process of the 

 squamosal as far as the articulation for the mandible. 



The Mandible (figs. 16 and 17). —The horizontal ramus of the mandible is very 

 stout, and its outer surface rounded from above downwards; anteriorly they unite in 

 a massive symphysis, the upper spout-like surface of which is continued forward by 

 the procumbent incisors, of which there are two pairs, the median of which are small 

 and laterally compressed teeth, the outer, large tusks. The ascending ramus is broad 

 and is inclined somewhat forward ; its anterior border rises from the outer surface of 

 the horizontal ramus in front of the hinder end of the molar series as in other 



* 'Geological Magazine,' dec. IV, vol. 8 (1901), p. 403. 

 t Geological Magazine,' dec. IV, vol. 9 (1902), p. 292. 

 VOL. CXCVI. — B. Q 



