2 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
With the exception of Plates IX and XI, the illustrations are by Mr. 
Bruce Horsfall, whose earnest cooperation has added in no small degree 
to whatever of interest and value the present memoir may contain. 
CLASSIFICATION OF THE SANTA CRUZ TYPOTHERIA. 
The Typotheria are grouped by Scott (1904, p. 590) as a suborder of 
the Toxodontia and may be defined as follows : 
Plantigrade or digitigrade mammals with pentadactyl * or tetradactyl 
feet, strongly interlocking carpus, without os centrale, and serial or slightly 
interlocking tarsus with hemispherical astragalar head. Dentition usually 
complete but tending toward reduction of lateral incisors, canine and an- 
terior premolars in specialized forms. Median incisor more or less 
enlarged and functional as a cropping tooth. Molars hypsodont, lopho- 
selenodont in crown-pattern, curving inward above and outward below. 
A clavicle is present in some forms. Femur with third trochanter. 
Fibula articulating with calcaneum. 
Two well marked families are recognizable among the Santa Cruz rep- 
resentatives of the suborder, for which the names Interatheriidae and 
Hegetotheriidae proposed by Ameghino may be retained, having priority. 
Each contains a large and a small form, of which the larger is the less 
specialized. The following key to the families and genera may be of use 
in facilitating the determination of new material : 
Order TOXODONTIA Owen. 
Suborder Typotheria Zittel. 
A. Family : Interatheriidae. Median incisors rooted ; third and fourth premolars not com- 
pletely molariform ; squamoso-mastoid region dilated and cancellous ; malar long and 
narrow, inclosed between temporal processes of maxillary and squamosal ; maxillary 
orbital ; carotid canal and foramen lacerum posterius fused ; tibia and fibula unfused dis- 
tally ; pes paraxonic, digits II and V equally reduced and small, digits III and IV large 
and of equal length ; astragalar trochlea bilaterally symmetrical ; no naviculo-calcaneal 
facet ; calcaneum with large fibular facet. 
I. Protypotherium. Dental formula |, |, |- in close series. Lateral incisors unreduced ; 
canine incisiform ; upper molars with deep internal inflection and slight antero-external 
ridges ; Mg- externally bilobate ; temporal bar of maxillary with slight descending process ; 
humerus with internal epicondylar foramen ; terminal phalanges laterally compressed, 
hoofs, with slight clefts in manus. 
{P. australe , praerutilum , attenuatum , Santa Cruz formation, Patagonia.) 
* Ameghino figures a pentadactyl manus in Pachyrukhos typicus ( 1889, PI. 13, fig. 14) and 
Typotherium (ibid., PI. 18, fig. 5). 
