No. 2.] THE HARD PARTS OF THE MAMMALIA. 213 
which display the involuted zygapophyses, viz.: the higher 
Diplarthra. These animals have other gaits also, but they can 
and very usually do, trot. The mammals which do not display 
this character of zygapophyses rarely or never trot. Thus the 
Carnivora pace and run. The Proboscidia pace. In some of 
the mammals of small size the trot may be observed, as in some 
Rodentia, but in some of these at least we can conceive that 
their insignificant weight does not permit sufficient strain to 
affect the form of the osseous material. From the standpoint 
here adopted, we must suppose the Creodonta to have used the 
trot as their principal gait. (See Plate IV., Fig. 1, lumbar verte- 
bra of Mesonyx.) 
I refer here to an exception to the above rule which I can- 
not satisfactorily explain. While Carnivora are pacers, the 
Canidz are to a considerable degree exceptions to the rule. 
They trot, or more frequently employ a gait between a pace 
and a trot; z.¢. in which the feet of one side are raised consecu- 
tively, yet not so as be synchronous with those of their diagonal 
opposites as ina trot. Since the Canidze are the ancestral Car- 
nivora, and nearest the Creodonta, (having close relations with 
the Miacidee), it is natural that they should retain more or less 
of the trot of that suborder. The involution of the zygapophy- 
ses has however entirely disappeared, a peculiarity which has 
yet to be accounted for. 
Although the cervical vertebrae of Mammalia undergo con- 
siderable torsion, their zygapophyses are flat. But the torsion 
has little force, from the fact that these vertebra sustain no 
lateral weight. 
Besides the zygapophyses, the vertebrata possess the zygo- 
sphen, the hyposphen, the episphen, and the zygantrapophysis ar- 
ticulations.! The zygosphen and hyposphen are nearly unknown 
among Mammalia, but the episphen and the zygantrapophysis 
are found in the American Edentata. The episphen is a pos- 
terior prolongation of the roof of the neural arch above the 
postzygapophyses, from which it is separated by a notch. It is 
really a portion segmented from the roof of a zygantrum. The 
zygantrapophysis is a prolongation of the superior surface 
of the postzygapophysis, and is directed posteriorly. In the 
American Edentata it bears an articular facet on its upper 
1 Proceedings Amer. Philosoph. Society, 1883, Pp. 545. 
