ORIGIN OF THE MAMMALIA, III. OCCIPITAL 
CONDYLES OF REPTILIAN TRI- 
PARIDE TYPE? 
HENRY FAIRFIELD OSBORN. 
Tue Huxleyan theory of the origin of Mammalia (Huxley, 
'80) from primitive Amphibia is still entertained by Howes 
(87) ; it has recently been revived by Hubrecht (97) upon the 
basis of the foetal membranes, and by Kingsley (99) upon the 
basis of the homology of the quadrate and upon other grounds. 
The position taken by the present writer ('98,'99) is that the 
weight of evidence favors the derivation of the Mammalia from 
some unknown member of the Anomodont reptiles (Therio- 
dontia) of Permian or lower Triassic age. 
One of Huxley's arguments for amphibian ancestry (80) was 
that the paired exoccipital condyles of the mammals were 
derived from similar amphibian structures. This is cited by 
Kingsley. The object of the present paper is to show that 
the mammalian occipital condyles arose from a reptilian tripar- 
tite type by the reduction of the median bastioccipital element and 
the expansion of the lateral exoccipttal elements. 
TYPES OF CONDYLES. 
The terms “ monocondylia " and **dicondylia " employed by - 
Cope fail to express the facts, for the junction of the basal 
elements of the skull with the atlas vertebra is of four types, — 
I, II, III, IV, as follows: 
I. Monocondylic, Typical 
Single ren Meg exoccipitals not entering into articula- 
Rep 
tion tilia (e.g., Alligator, Ichthyosaurus) 
1 Presented before ace of Zoólogy, American Association for the Advance- 
ment of Science, June, o. Contributions I and II upon the Origin of 
Mammals are cited in ^ bibliography. - 
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