i8 
PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
former. The depression inclosed by these elements rapidly disappears, 
leaving no trace of enamel lakes. The posterior lobe of the third molar 
differs from that of the second only in its greater length antero-posteriorly. 
Milk Dentition (PI. V, figs, n-14.) — The milk-premolars may be 
readily recognized by the presence of roots. So far as can be ascertained, 
the order of replacement seems to be the normal one. In the dental 
series figured on Plate V, figures 1 1 and 12, the first and second permanent 
molars are in place and already somewhat worn, the first more so than 
the second. The third is just appearing below the alveolar border. An- 
terior to these are three double-rooted deciduous teeth. Of the two 
roots the anterior is the larger. Dp A is almost completely molariform, 
showing the same arrangement of external and internal grooves as in the 
permanent molars. Dp- and dp- are deeply grooved externally, as in their 
permanent successors, but the groove is continued but a short distance 
above the alveolar border up the outer side of the anterior root. In- 
ternally, the crowns are slightly grooved, producing the same separation 
into anterior and posterior lobes as in the permanent premolars. P x seems 
to have had no deciduous predecessor. The crown-pattern of the decidu- 
ous premolars (PI. V, fig. 1 1 a) is the same as that of their permanent suc- 
cessors, so far as can be determined from the partly worn teeth available 
for comparison. As already explained, this consists of a pair of internal 
crescents and a broadly concave ectoloph. The depressions inclosed be- 
tween the crescents and the ectoloph are shallow and soon disappear. 
Only in little worn teeth are isolated lakelets, due to the partial oblitera- 
tion of these depressions, observable on the triturating surface of the crown 
(PI. V, fig. 11). Sufficient material is not available for determining the 
order of replacement of the incisors and canine. 
In the lower jaw, the eruption of the true molars follows the same order 
as in the superior series. Dp^ is identical in pattern with the molars, while 
Dp- 3 - and Dpa resembles their permanent successors. As in the superior 
series, P T appears to have been without a deciduous predecessor, as the 
little-worn tooth is hypsodont. The order of replacement of the milk den- 
tition is believed to have been from behind forward, judging from the fact 
that P^oes not appear until some time after the eruption of the third and 
fourth premolars (PI. V, fig. 15). 
Skn/l (Plates III; V, fig 21). — There is little difference, apart from 
size, between the skulls of the various species, the specific characters de- 
