196 B. NATURAL HISTORY. 
* Malar portion of zygomatic arch absent. 
Lacertilia Varanide.. 
** Malar portion present. 
Lacertilia in general. 
Anomodontia. 
Sauropterygia (% all). 
Testudinata in general.* 
aa. Without postorbital bone. 
* Malar portion wanting. 
Batrachia Urodela Pleurodelide. 
Pythonomorpha. 
** Malar portion present. 
Batrachia Gymnophiona. 
Mammalia Quadrumana, Artiodactyla, Perissodactyla (part). 
2. Without postorbital arch. 
Mammalia Carnivora Proboscidia Perissodactyla (part), Cetacea, Rodentia, 
Edentata (part), Monotremata. 
From the above table, it will be observed that each class, and 
sometimes single orders, present many or several of the vari- 
ous types of structure of the arches. These arches are more or 
less protective or fixative in their use; that is, they protect the 
orbit, the temporal muscle, or the oral cavity, or fix the quadrate 
and prevent its motion. As adaptive characters, they are thus 
those which define very subordinate representatives of all the 
orders. 
From want of analysis, the proper determination of the arches 
has not always been made, and the identification of the component 
and adjacent bones vitiated. This is no doubt owing to the fact, 
that in many Reptilia, where the orbits are large, and the temporal 
fossa small, —e. g., Ichthyopterygia, Crocodilia, etc.,— the zygo- 
matic arch makes a strong sigmoid flexure, leaving the quadrato- 
jugal to take the more direct course to its terminus. Thus Owen 
(Paleontology) homologizes the quadratojugal arch of Ichthyo- 
saurus with the zygomatic of Mammals, and the true zygomatic 
with the temporal fascia ofthe same. In the same way (i. ¢., p. 
210), he homologizes the quadratojugal arch of Nothosaurus with 
the zygomatic, thus: “‘ The lower one (i. ¢., arch) is formed by the 
malar (27) and squamosal (28), the latter answering to the true 
zygomatic arch in Mammals.” The figures obviously refer, in the 
cut, to the malar and quadratojugal bones; while the “mastoid” 
* The postorbital is prolonged so far downwards in Chelone and Chelydra, as 
to look like a quadratojugal. 
