418 
PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 
notch has disappeared and the protoconid and paraconid are of the same 
height, forming an elevated trenchant blade. 
Owing to the absence of transitional forms in the Santa Cruz fauna, so 
far as known, it is less easy to trace the development of the peculiar 
notched and fluted sectorials of Abderites, which are to be regarded as 
highly specialized structures adapted to a piercing habit, suggesting that 
the animal fed on the eggs of birds. The loss of the metaconid is a 
further adaptation toward the perfection of the piercing function. The 
derivation of the sectorial teeth in Abderites from the tuberculo-sectorial 
type of molar characteristic of the Caenolestinae is indicated by the broad 
heel, and by the additional fact that the remaining molars, although some- 
what less lophodont than in the Palaeothentinae, retain both the paraconid 
and metaconid as distinct cusps. 
A satisfactory discussion of the derivation of the upper molar patterns 
in the Caenolestidae is at present impossible, owing to a lack of material 
illustrating the upper dentition in many of the genera, especially in the 
more primitive forms. 
Less uncertainty exists regarding the lower molars. In the Palaeo- 
thentinae, lophodont molars have been developed from teeth of the primi- 
tive tuberculo-sectorial type, shown in Halmarhiplms , by the formation of 
cross crests uniting the cusps of the talonid and heel. In the buno-loph- 
odont molars of Abderites , all the cusps of the original tuberculo-sec- 
torial crown have been retained, except in My. The loss of cusps in this 
tooth has already been discussed. 
So little is known of the skull in the majority of the Caenolestidae that 
any attempt at a discussion would resolve itself into a repetition of 
Thomas’s excellent description of the skull of Ccenolestes (1895). A de- 
scription of an incomplete skull of Palceothentes will be found on a later 
page, to which, and to the accompanying illustrations of the skull of 
Ccenolestes (PI. LXIII, figs. 14-14$) the reader is referred. 
Little is known of the podial structure of the Caenolestidae. Ameghino 
(1894, pp. 80, 81) describes the feet of the Santa Cruz representatives of 
his suborder Paucituberculata (= the Caenolestidae) as follows: “The four 
limbs were almost equal in length, but the hind feet were longer than the 
fore. They were plantigrade, with five toes on the hind feet and probably 
also on the fore feet, with all the toes well developed and without the 
least trace of syndactyly.” 
