210 COPE. [Vot. III. 
task requires much more force for its execution both in the fore 
legs and feet in holding the bones to the earth, and in the neck 
in dragging them asunder; while the task of comminuting them 
is performed by the large sectorial molar teeth, the most power- 
ful and massive in the entire order of Carnivora. 
3. THE VERTEBRAL ARCHES. 
The mechanical cause of the origin of neural spines may be 
traced to the strains upon the vertebral axis caused by a pri- 
mary dorsal fold or fin; and in later and terrestrial types, to 
the strains imposed by the presence of the dorsal muscles. The 
existence of diapophyses may be perhaps assigned to the strains 
from the two primitive lateral folds from which the pectoral and 
ventral fins were evolved. The zygapophyses occupy the space 
on the vertebra, between these two fulcral points, and constitute — 
an important element in the interlocking of the vertebra. Of an 
efficient mechanical cause for their origin we are ignorant, and 
the search for it would more appropriately belong to the inves- 
tigation of the lower vertebrata than to the present paper. I 
only recall here that the crests of the zygapophyses are the seat 
of the insertions of a part of the mz/tifidus spine muscle, and 
receive its strains. 
Mammalia present important peculiarities of the zygapophy- 
ses. That these are well-marked may be derived from the 
accompanying plates III.-VI. The primitive form of simple 
flat surfaces prevails in the lower orders, offering only differ- 
ences in their greater or less obliquity. In Manis, the Creodonta, 
and the Ungulata, this obliquity assumes an interlocking char- 
acter. The prezygapophysis becomes concave in transverse 
vertical section, forming an anteroposterior canal, while the post- 
zygapophysis becomes a partial anteroposterior cylinder, which 
fits within the former. In the Artiodactyla the structure is fur- 
ther modified in various genera. In Sus and Capra as extreme 
examples, the postzygapophysis becomes sigmoid in transverse 
section, its external surface displaying a longitudinal cylinder 
below, continuous with a groove above, the whole surmounted 
by the overhanging edge of the roof of a process, or episphen. 
Since these structures appear only in the lumbar region, where 
motion is possible owing to the absence of rib connections, and 
