RODENTIA. 



RODENTS. 



Characters :— These are mammals which have no canines, 

 and in which the most prominent and unmistakable character 

 is formed by the large, curved, chisel-shaped, rootless incisors, 

 which often have their anterior surfaces yellow or red, and are 

 separated by a long space or diastema from the cheek-teeth, 

 the crowns of which may be tuberculated or laminated. In the 

 skull there are zygomatic arches, the central portions formed by 

 the malar bones, and auditory bullae ; the orbits communicate 

 freely with the temporal fossae; the lachrymal foramina are always 

 within the margin of the orbits, and there are no postglenoid 

 processes to the squamosals. In the mandible the condyle is 

 elongated from before backwards, so that the jaw can be moved 

 backwards and forwards ; the symphysial portion is abruptly 

 narrowed and rounded in support of the large incisors, the 

 coronoid process is small and the angular part greatly developed. 

 The radius and ulna are separated ; the ischia and pubis are 

 well developed and meet in a long symphysis. The mouth is 

 divided into two cavities ; the testes are abdominal or inguinal, 

 becoming more prominent during the rutting season ; there are 

 prostatic glands and, except in the Duplicidentata, vesicuke 

 seminales ; the uterus is two-horned ; the placenta is discoidal 

 and deciduate ; the cerebral hemispheres are smooth, and do 

 not extend backwards far enough to cover any part of the 

 cerebellum; the temporal muscle is comparatively small, the 

 masseter large and double. 



This great and well-defined order, which in number ol 

 species far exceeds all the others, dates from the Lower Eocene 



