946 
almost be termed tripartite. 
THE AMERICAN NATURALIST. 
[Vor. XXXIV. 
This gradation between a tripartite 
or transitional dicondylic condition and the * typical dicondylic ” 
condition in the mammals is very clearly shown in the second 
series of diagrams (Fig. 2), which show 
these structures as seen ee below or 
in palatal view. 
I am indebted to Mr. B. Arthur 
Bensley, Fellow in Zoólogy, Columbia 
University, for examining a large series 
of skulls in the American and Yale 
Museums (7 Monotremata, 12 Marsu- 
pialia, 15 Rodentia, 12 Carnivora, 7 In- 
sectivora, 3 Cheiroptera, 4 Lemuroidea, 
3 Anthropoidea, 3 Artiodactyla), with 
the following results : 
Monotremata : The thickness of the 
basioccipital plates is a conspicuous 
feature of all monotreme skulls. (Yale 
Museum, Nos. 244, 300.) In three 
skulls of Echidna the exoccipital con- 
dyles are joined by a broad cartilaginous 
(basioccipital) bridge, very conspicuous 
after soaking in water (Fig. 3, 447, 4?) ; 
in two skulls the condyles are joined 
by a narrow ridge; in two others (4°) 
the condylar ridges are discontinuous. 
In Ornithorhynchus there is little varia- 
, tion in the condylar region (Fig. 3, 5’, 
B?); the condylar ridge is continuous 
across the basioccipital region, the artic- 
ular cartilages being slightly separate 
only when viewed from below. So far 
as this positive evidence goes, it is con- 
firmatory of the theory of tripartite origin. 
Marsupialia: The exoccipital condyles are distinct and sep- 
arate in all forms examined except in Dasyurus, where there is 
a slight tendency to bridge the basioccipital (D. maculatus) ; 
this negative result is in keeping with the writer's theory of 
the zon-primitive character of the marsupials. 
