316 THE AMERICAN NATURALIST. [VoL. XXXII. 
ectopterygoids; humerus with an entepicondylar fora- 
men; digital formula of fore foot, 2, 3, 3, 3, 3 phalanges.” 
No definition could be clearer, and upon the following page 
Owen suggests the hypothesis that these forms may have given 
rise to the mammals “ by secondary law, the mode of operation 
of which we have 
still to learn.” } 
This definition 
was subsequently en- 
larged by Owen him- 
self, and has been 
extended by Seeley. 
So that now this or- 
der includes forms 
having great diver- 
sity in their denti- 
tion, but apparently 
related in their oste- 
ological characters. 
Thus, says Seeley 
(1895, 1, p. 997), the 
Theriodontia as orig- 
inally defined includ- 
ee ore iews of the skull of Dicynodon, ed: first, the group of 
she owin elements as interpreted by Professor ‘Seeley. é . 
Note piamen the exposure of the vomer, the large exten- animals with skulls 
sion of the squamosal, the pre- and postfrontals, i single formed on the type 
q ll y bar. È 
of Lycosaurus? with 
simple pointed teeth; second, the group with skulls formed on 
1 Quar. Journ. Geol. Soc., pp. 95-101, May, 1876. 
2 With this group of theriodonts Case, in a recent paper (American Naturalist, 
February, 1898, p. 73), also associates the Lycosauria; Lycosaurus being a type — 
which furnishes a aoa from the supposed fusion of the upper and lower 
temporal arches into the single zygomatic arch of the Mammalia, as shown in the 
porte 3 eas 
Cynod bask covered by supporting bones. Teeth showing small 
lateral evenly Arches more closely approximated than in Procolophonia. 
Lycosauria: Quadrate small, covered by supporting bones. Skull depressed. 
Teeth with well-developed tubercles. Arches unite 
apperse Quadrate very small and inclosed in squamosal. T eeth tubercu- 
. Arches united. 
