276 PALEONTOLOGY. 



last, in so far as can be predicated from what remains, must have been very regularly rounded. 

 The left branch is quite short, being broken just in front of the first molar tooth. The dimen- 

 sions of the fragment are as foUows : 



Inches. 



Length of specimen --------- 14.50 



Length from symphysis to base of coronoid process - - - 12.50 

 From symphysis t|o base of first molar - - - - - 5.00 



Space between right and left branches of jaw - - - - 2.75 



Symphysis from '"efore backwards - - - - - - 4.75 



Length of alveolar portion - - - - - - -6.00 



Width of gutter at extremity of symphysis - - - - 0.75 



Transverse thickness of jaw at base of coronoid - - - 4.75 



Height of jaw in front of coronoid - - - - - - 4.25 



Height of jaw in front of first molar ----- 5. 00 



Transverse thickness of jaw at coronoid - - - - - 4.75 



Transverse thickness of jaw at base of first molar - - - 2.50 

 The inner face of the jaw is nearly vertical, and is almost exactly parallel to the median line, 

 except posteriorly, where it diverges from it and becomes convex. The lower edge of the jaw 

 is horizontal, but the upper or alveolar portion ascends rapidly from behind forwards till it 

 reaches the anterior extremity of the first molar, where it becomes continuous with a sharp 

 ridge having a slightly serpentine outline, and converging as it descends forwards towards a 

 similar one from the opposite side ; and the two include between them a gutter or channel, 

 which is met with under various modifications in both Mastodons and Elephants. This channel 

 is continued, gradually diminishing, to the most prominent part of the chin, where it termi- 

 nates in a rounded depression ; but a small, narrow groove extends from this last about three 

 inches along the under side of the symphysis. When seen in profile, the symphysis forms a 

 slightly depressed beak, with a regularly rounded extremity. This part in other Mastodons is 

 usually quite jjointed, the symphysis having the appearance of having been obliquely trun- 

 cated. The greater elevation of the front part of the alveolar portion is doubtless to be attrib- 

 uted to the worn condition of the tooth ; the former being generally built up as the latter wears 

 away, and thus keeping the grinding surface constantly on the same level. The canal for the 

 mandibular branch of the fiftli pair of nerves is about one half of an inch in diameter at its 

 posterior portion, lies quite near to the inner face of the bone near its lower border, and run- 

 ning parallel to it till it reaches a point near the first molar, where it passes obliquely forwards 

 to the outer surface, on which it opens by a single foramen just in front of the tooth, and mid- 

 way between the upper and lower edge. 



The teeth consist of two molars in place, and of a fragment of a third which is imperfectly 

 developed, and the points of which had not yet risen above the edges of the alveoli. 



The anterior tooth, (PI. xii, Figs. 1 and 2, IV,) which, from the existence of an anterior and 

 posterior supplementary ridge or talon, may be regarded as the fourth in the complete dental 

 series, has the crown worn down quite near to the base of the ridges, traces of all of which — 

 viz : the three principal and the two supplementary ones — still remain. The dimensions of 



this tooth are as follows : 



Inches. 

 Length - - - .----.- 2.75 



Breadth in front, at anterior ridge ------ 1.50 



Breadth posteriorly, at third ridge - - - - - - 1.75 



The inner side of the crown is less worn than the outer, so that nearly all the traces of the 

 transverse ridges have disappeared externally ; but on the inner side they are represented in 

 transverse sections, which have the characteristic trefoil-shajjed appearance. At the bottom of 

 the interval, between two adjoining ridges, are converging grooves of enamel which unite in a 

 couiiuon uhaunel. 



