LEPUS CUNICULUS 291 



The bringing of the hallux to the inside of the hind limb is not 

 accomplished in the same way, but this is done by the rotation of 

 the limb as a whole, whereby the original dorsal border becomes 

 the anterior border, but there is no extra twisting of the lower part 

 of the limb. 



This ideal type of limb is only met with in a few primitive 

 animals and in all others a certain, often very considerable, amount 

 of modification has occurred. One of the first changes is a fusion 

 or loss of certain of the carpal and tarsal elements, and another 

 common one is a loss or reduction of the digits. 



The Skull of a Mammal The Dog, Canis familiaris. 



For the study of a typical mammalian skull we may take 

 that of the dog which, although not ideal, is fairly generalised and 

 readily obtainable. While it is to the dog's skull that the details 

 of the following description apply, it is well to bear in mind that the 

 skull of all the higher Mammalia is constructed upon the same 

 general plan and the differences are mainly small points, and 

 particularly in the relative sizes of the parts. As in the frog, the 

 skull of the mammal is first laid down in cartilage. In the adult 

 it comes to consist of certain parts of this chondrocranium that have 

 ossified and so became cartilage bones, to which have been added 

 other membrane bones laid down in the surrounding tissues. Again, 

 in Canis as in Rana the entire skull consists of the cranium or brain 

 case, the olfactory, optic and auditory capsules, the upper and 

 lower jaws and the hyoid apparatus, and to laciliate description 

 we may deal with these various parts separately. The various 

 bones in an old skull are closely joined together by small inter- 

 locking projections so that the line of union is indicated by wavy 

 lines termed sutures, or it may be that they are so closely fused as 

 to be indistinguishable. For purposes of study, therefore, it is well 

 to examine a moderately young specimen. 



Cranium. The cranium is a large, hollow box somewhat oval in 

 shape and occupying slightly more than the posterior half of the 

 skull. The floor or base is more or less straight, while the roof is well 

 arched. In the hinder part of adult skulls we find a prominent 

 ridge in the mid-dorsal line ; it is termed the sagittal crest and 

 furnishes attachment for the large temporal muscles moving the 

 lower jaw. The bones composing the cranium will be found to 

 fall into three segments corresponding with three median, unpaired 

 bones, namely, the basi-occipital, the basi-sphenoid and the pre- 

 sphenoid, which together form the basi-cranial region of the skull. 



The occipital segment is composed of cartilage bones of which 

 the mid- ventral is the basi-occipital, a flat plate at the hinder end 



