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PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 
Stage i. — Milk-dentition of dif ; dc-f ; dpt. The dp are the first teeth 
erupted and the last are the external incisors and canines ; all the teeth in 
continuous series. 
Stage 2. — Milk-teeth in function and no permanent tooth yet visible. 
In the mandible the four internal incisors are of equal size and the two 
external ones a little larger, and all the teeth have open roots. The two 
rami of the mandible are in contact, but have not yet begun to ankylose. 
Stage j. — Mf in function, but mf not visible. The anterior part of the 
skull is elongated, producing short diastemata on each side of the upper 
canine, and the first upper incisors, diminished by wear, are separated by a 
space. Lower teeth still in continuous series ; roots are closed in di T -dp T 
and roots begin to appear in dp^- and ^ : the germs of most of the per- 
manent ante-molars are clearly visible, the two rami of the mandible are 
ankylosed, and the sagittal crest begins to appear. 
Stage 4. — Mi considerably worn and mf coming into use; milk-teeth 
much abraded and dpi and i with four separate roots ; germs of the per- 
manent incisors well developed and visible in the sides of the jaws. 
Germs of pf quite large and placed above dp x and below dpy. The 
masticating surface of all the teeth in use has the appearance of that 
belonging to an adult animal. The mandible described and figured by 
Owen as N. imbricatus is in this stage of dental development and was 
mistakenly believed by him to be an adult individual. 
Stage 5. — Mf well abraded; none of the milk-teeth yet replaced. 
Germ of m- small and placed in the upper part of the maxillary against 
the base of m- ; that of m3, also small, is placed in the base of the ascend- 
ing ramus ; germs of pf very small and hardly visible. This stage cor- 
responds to Nesodon as defined by Mercerat. 
Stage 6. — Germs of mf quite large, but not yet erupted; replacement 
of incisors begins : germs of pf very small. 
Stage 7. — Mf erupted, but not yet worn; germs of pi larger, dc, 
dpi-! shed and replaced. 
Stage 8. — Mf enters into use; dpi replaced by pf. 
Stage 9. — All the permanent cheek-teeth in use and have open bases. 
I 1 much broader and thicker than i~, which is thin and pointed and has 
its tip nearly on a level with that of i-. Cheek-teeth gradually increasing 
in size ; m- with relatively small grinding surface, and mi but slightly 
larger than mi; upper molars with cross-section a parallelogram. This 
stage corresponds to Adelphotherium Mercerat. 
