The Rodent ia in Evolution. 49 



readily be understood if we suppose they are descended 

 from these. They are all much alike osteologically, the 

 beaver being the only one at all widely divergent, as can be 

 seen in the case of the ground squirrel, the flying squirrel, 

 and the beaver by a comparison of figures 2, 4 and 5. 



The Hystricomorpha or porcupine kind are almost 

 strictly confined to South America, the chief exception being 

 the true porcupine of Europe, which furnished a figure for 

 Shakespeare in Hamlet. The most of the family are not 

 spiny, nor do they correspond with the correct notion of a 

 porcupine, but they present structural features which dis- 

 tinguish the sub-order. The skull is broad in the frontal 

 and parietal region, but the molar bone is short and does 

 not run up to join the lachrymal, the zygomatic arch is 

 short and the infraorbital foramen is very large (as in the 

 beaver only among Sciuromorphs). The molars are never 

 five in number and are in many less than four, in some three 

 or even only two. The|;e are usually four sacral vertebrae 

 and sometimes (Paca) five. The number of toes is rarely 

 five and sometimes only three and the metatarsals are never 

 fused. The tibia or fibula are always distinct. These char- 

 acters are easily derivable from the Sciuromorph type — the 

 reduction of number of teeth, the incorporation of more 

 bones in the sacrum and the reduction in the number of toes 

 are all quite easily possible from Tamias as a starting point. 

 The various creatures of the suborder are but little known 

 except to travelers or special students. Some of them are 

 the Deg^a of Chili of the size and habits of a rat ; the plate 

 beaver of Brazil, semi aquatic, with cylindrical scaly tail but 

 soft wooly hair; the porcupines arboreal often prehensile 

 tailed creatures; the chinchilla a terrestrial and fossorial 

 creature of Chili and Peru; the vischaca of Argentine Re- 

 public burrowing so extensively as to make the ground 

 unsafe for man or beast ; the paca and capybara, the latter 

 as large as a Newfoundland dog and weighing 100 pounds, 

 a terrestrial animal but perfectly at home in the water; the 

 Pategonian ''guinea pig" or cavy a terrestrial and fossorial 

 animal. 



The Myomorpha or mouse-kind is the largest suborder 

 of Rodentia and the most universally distributed being^ 



