by great thickening of the cranial bones, as is true of elephants and rhinoceroses, and in them sufficient surface for attachment is afforded to the muscles without the development of a crest. [Illustration: FIG. 8.—Skull of Wolf, top view. _P.Mx._, premaxillary. _Na._, nasal. _Ma._, malar or jugal. _L._, lachrymal. _Fr._, frontal. _Sq._, squamosal. _Pa._, parietal. _S.O._, supraoccipital.] [Illustration: FIG. 9.—Skull of Wolf, view of base. _P.Mx._, premaxillary. _Mx._, palatine process of maxillary. _Pl._, palatine. _Fr._, frontal. _Pt._, parietal. _Ma._, malar or jugal. _Sq._, glenoid cavity of squamosal. _B.S._, basisphenoid. _B.O._, basioccipital. _Ty._, tympanic (auditory bulla). _p.oc._, paroccipital process. _con._, occipital condyle. _S.O._, supraoccipital.] The structure of these cranial bones, more particularly of the parietals, is subject to important changes; in most mammals they are of moderate thickness and have dense layers, or “tables,” forming the outer and inner surfaces and, between these, a layer of spongy bone. In many large mammals, however, especially those which have heavy horns or tusks, the cranial bones become enormously thick and the spongy layer is converted into a series of communicating chambers, or _sinuses_, the partitions between which serve as braces, thus making the bone very strong in proportion to its weight. Sinuses are very generally present in the frontals and communicate by small openings with the nasal passage, even in genera of moderate size and without horns or tusks. The frontals form the roof of the eye-sockets, or _orbits_, and usually there is a projection from each frontal, which marks the hinder border of the orbit and is therefore called the _postorbital_ process. The roof of the facial region is made by the _nasals_, which are commonly long and narrow bones, but vary greatly in form and proportions in different mammals; in those which have a proboscis, like tapirs and elephants, or a much inflated snout, such as the Moose (_Alce_) or the Saiga Antelope (_Saiga tatarica_) the nasals are always very much shortened and otherwise modified in form.