ORDINAL TYPES OK MOLARS: SIRENIA, KT< '. 



1 89 



me- 



Sirenia are an aquatic offshoot from the Ungulata. I>e lUainville 



first made the bold suggestion that they were related to the Proboscidea, 



and in 1002 Andrews again directed attention 



to the numerous anatomical similarities of these 



two groups, reinforcing them by the new evidence 



offered by the ancestral Mcerithcriitui, which 



closely resembles Prorastoma not only in the 



teeth but in the humerus. Lydekker, on the 



other hand, in 1892, pointed out the likenesses 



between the third and fourth upper milk molars 



of Prorastoma vero/n'ti*' and those of the seleno- 



dont Artiodactyl Merycopotamus dissimilis. 



Of the affinities of the Sirenia and Ungulata in general, as 

 expressed in the table on p. 75, there can be little doubt, but for 

 their more specific relationships we must wait for additional evidence. 



Fie;. 191. Unworn molar of 

 American Manatee (Ti-ichechus 

 inmiiih'x), showing "masked 

 selenodonty." x^. 



SPECIAL REFERENCES. 



Andrews, C. W., "On the Evolution of the Proboscidea," Phil. Trails. Roy. 

 Lond., Ser. B, Vol. 196, 1903, pp. 99-118. 



Lydekker, K., "On a remarkable Sirenian Jaw from the Oligocene of Italy and its 

 bearings on the Evolution of the Sirenia," Proc. Zool. Soc. Lond., Febr. 2, 1892, 

 pp. 77-83. 



SOUTH AMERICAN UNGULATES. 



The Ungulates of South America include the highly specialized 

 Typotheria, Homalodotheria, Toxodontia, and Astrapotheria, in all of 

 which the teeth present the extreme of lophodont modification and of 

 elongation or hypselodontism. Close analogies are found among the 



ni.3 



Fio. 192. Upper cheek teeth of Prottrothi rium sp., a primitive Litoptern from the Santa Crux 

 Formation, Middle (?) Miocene, Patagonia. Compare the somewhat similar buno-lppho-selenodont 

 molars of Palaotherium, Protorohippus (Fig. 100), Sairftu.tliei imn (Fig. l.v'O, Mi nim-ntln r'nun (Fig. 

 183), SchizotJicrium (Fig. 184) and C'cenotherium, familv Anplotheriidse, order Artiodactyla. 



teeth of the Equido_ j and Ehinocerotidse, and especially of the Amyno- 

 dontidfe. Naturally there is little trace left of the archaic and simpler 

 constitution of the teeth in these highly specialized crowns. 



It is very suggestive, however, that the most primitive of the 

 Litopterna, the fifth of these South American orders, retain unmis- 

 takable indications of a primarily triangular crown pattern. In 



