38 Field Museum of Natural History — Zoology, Vol. XL 



Tail vertebrae or Tail — Distance from point where it joins the body 



to the tip (not including the hairs which may extend beyond). 

 Hind Foot — Distance from the heel (tarsal joint) to the end of the 



longest claw. 

 Ear — Distance from point where it joins the skull to the tip (this is 



the method followed in the present work; some persons measure 



from the notch to the tip) . 



LIFE ZONES. 

 Naturalists have learned that the surface of the earth is divisible 

 into regions representing the distribution of the various forms of animal 

 and plant life, which, it has been found, is governed by variations in 

 temperature. Dr. C. Hart Merriam says: '^ Investigations conducted 

 by the Biological Survey have shown that the northward distribution 

 of terrestrial animals and plants is governed by the sum of the positive 

 temperatures of the entire season of growth and reproduction, and that 

 the southward distribution is governed by the mean temperature of a 

 brief period during the hottest part of the year." 



The North American continent is divided into three great primary 

 regions: (i) The Boreal Region, comprising the Arctic, Hudsonian, and 

 Canadian zones; (2) The Austral Region, which contains the Transition, 

 Upper Austral and Lower Austral zones ; and (3) The Tropical Region, 

 the last being represented in the United States only in southern Florida 

 and a portion of Texas. 



The more humid portions of the Austral zones lying east of the 

 Great Plains (approximately east of the looth meridian of longitude) 

 are divided into faunal areas, or faunas, known respectively, as the 

 Alleghanian, Carolinian and Austroriparian faunas. 

 The Arctic Zone — This zone lies far north, beyond the limit of tree 

 growth, but is also represented in more southern localities on high 

 mountains above the line of timber, where it is designated as Arctic- 

 alpine. 

 Hudsonian Zone — This comprises the most northern forested regions 

 of the continent, and is largely covered with firs and spruces. It is 

 represented in more southern localities on the upper wooded slopes 

 of high mountains in the United States, where similar climatic 

 conditions obtain, and is there termed Hudsonian-alpine. 

 Canadian Zone — This comprises the more southern portion of the con- 

 iferous forest regions of Canada and the northern parts of Wisconsin, 

 Michigan and New England. Under the name of Canadian-alpine it 

 also includes portions of mountains in the United States and Mexico, 

 situated at an altitude where similar climatic conditions exist. 



