128 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
Owing to the very early replacement of dp T by p x , I have seen no ex- 
ample of the former. The second milk-premolar resembles p x , but in N. 
imbricatus a small separate pillar stands at the entrance of the anterior 
valley and a spur from the anterior crescent almost closes the posterior 
valley, which, when well-worn, becomes a lake in the grinding surface of 
the posterior crescent. The third and fourth of the series (dp x and T ) 
resemble molars except in their lower crowns and the earlier formation 
of roots. 
Pre-lacteal Dentition. — Ameghino has announced (’02, 80; ’04, 10) the 
exceedingly surprising discovery that in Nesodon and Adinotherium a 
third functional dentition, the pre-lacteal, is present. Traces of such 
a dentition had already been observed in the embryos of certain mam- 
mals, but no case was known in which these teeth were fully developed 
and functional, and to find them in so advanced and specialized a group 
as the toxodonts and at so late a geological date as the Santa Cruz epoch, 
is indeed most extraordinary. According to Ameghino’s latest account: 
“ L’avant-premiere serie des Nesodontides est constitute par trois incisives, 
une canine et trois molaires de chaque c6te, qui sont remplacees par les 
memes dents de la premiere serie” (’04, 11). “Chez Nesodon et Adino- 
therium la denture definitive comprend le nombre complet de sept molaires, 
dont les quatres anterieures sont des rempla^antes et les trois dernieres 
des persistantes de la premiere serie. La premitre strie est constitute par 
les quatre caduques anttrieures et les trois persistantes posttriures, les 
sept molaires, restant en fonction en merne temps durant une certaine 
ptriode de la vie de ces animaux. A un age moins avanct, avant d’entrer 
en fonction la premitre persistante, la strie n’ttait constitute que par les 
quatre caduques. Les caduques, leur tour, ont ttt les remplacantes 
d’une strie anttrieure, Tavant-premitre strie qui ne comprenait que trois 
molaires correspondantes a la premitre, deuxitme et troisieme molaires. 
Done, les caduques 1 a 3 ttaient prtctdtes par les avant-caduques cor- 
respondantes mais la quatrieme caduque n’ttait pas prtctdte d’une avant- 
caduque ” (1. c., pp. 14-15). 
To this description I can add but little, as the material in the Princeton 
and New York collections exhibits hardly any remnant of the pre-lacteal 
series, but in one specimen each of Nesodon and Adinotherium (see text- 
fig. 39, p. 202) the pre-lacteal upper tusk has persisted alongside of the 
permanent i- and is much smaller than di~ and of entirely different shape, 
