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ANNALS NEW YORK ACADEMY OF SCIENCES 
and Triassic of South Africa. The recent discoveries by Broom have 
brought strong support to the view that these forms are allied to the stem 
of the Mammalia, from which they differ chiefly in retaining many primitive 
reptilian characters, notably in the inferior surface of the skull and in the 
lower jaw. The quadrate, articular and angular bones were smaller in the 
later than in the earlier Theriodonts, and there is much to prove that the 
mandibulo-squamosal joint in the mammals is a neomorph, formed con¬ 
comitantly with the reduction of the quadrate, articular and angular. The 
mammals may have arisen from some small insectivorous Theriodont allied 
to Galesaurus. 
The second stage in the evolution of the mammals is represented by the 
American Triassic genera, Dromatherium and Microconodon, both known 
only from the lower jaw. Here the ascending corono-eondylar ramus of 
the dentary had grown backward into a small but distinct mandibular 
condyle. The shape of the dentary seemed to indicate that the articular 
and angular bones might still have been retained in a reduced condition. 
Resemblances to the Theriodont Tribolodon of South Africa, in connection 
with the exceedingly primitive features of the dentition, supported the 
inference that these minute insectivorous forms were near the border land 
between reptiles and mammals. 
The third stage is represented to some extent by the Monotremes of 
Australia. In the existing genera the skull is very aberrantly modified, but 
they have retained oviparous habits and a very lowly type of brain, while 
the shoulder girdle, humerus, carpus, tarsus, etc. are of modified Theriodont 
type. Although not known before the Pleistocene, the Monotremes are thus 
of an exceedingly archaic type which probably dates back in many characters 
to the Triassic. 
The fourth stage is typified by the celebrated genus Amphitherium from 
the Middle Jurassic (Stonesfield Slate) of England, known only from the 
lower jaw. The dental formula could give rise by reduction to either the 
Marsupial or the Placental types, the cheek teeth are very primitive both in 
form and number, the angle in one species is partly inflected; the habits 
were probably insectivorous. 
The fifth stage is partly typified by the smaller insectivorous Marsupials, 
especially the Murine Opossum ( Marmoset) of South America. All Marsu¬ 
pials have departed from the primitive Marsupio-Placental stem in the 
partial suppression of the milk dentition, loss of premolar 2, predominence 
of the yolk-sack placenta, peculiar modifications of the reproductive organs, 
etc. On the other hand the Polyprotodont Marsupials, especially Marmoset, 
retain certain primitive characters which have been more or less lost in the 
Placentals, especially the primary adaptations for arboreal habits, the general 
architecture of the skull, the characters of the pelvis, astragalus, etc. 
