892 > The American Naturulist. [November, 
In comparing the Holarctic with other realms, I have already 
referred to the number of types which it possesses in common 
with the Ethiopian, not found in the Neotropical. It has also 
several in common with the Neotropical, which do not occur in 
the Ethiopian. These are the Arciferous Batrachia, the Cro- 
talid snakes, and the deer (Cervide). The Medicolumbian 
division of the Holarctic shares other forms with the Neotropi- 
cal. These are Didelphyidz and Procyonide among Mammalia; 
Tyrannid, Icterid and Tanagrid birds; Kinosternid tortoises 
and the Arciferous Batrachian family Hylide. 
Some of the forms of the Holarctic region are not uniformly 
distributed over it. Thus the Ginglymodous and Spatulariid 
fishes only occur in the eastern parts of the eastern and western 
continents. The same is true of the Silurid genus Amiurus 
and the Loricate genus Alligator. The Crotalid snakes are 
not found in the western parts of Eurasia. The Batrachian 
Cryptobranchidez have the same distribution. 
Il. Tot MepicotuMBIAN REGION. 
This region was formerly included-in the Nearctic of Sclater, 
and the area thus constituted has the following geographic 
boundaries. To the south it includes the plateau of Mexico, 
including the central valley. The Neotropical area bounds 
it to the east and west, occupying the low-lands or Tierra Ca- 
liente to a point 150 miles south of the Rio Grande on the east, 
(Townsend, Texas Academy of Science, 1895, p. 87), and to 
Mazatlan, or some point not far from it, on the west. The high 
land of Oaxaca is its extreme southern outpost. Its northern 
boundary is thus described by Merriam.’ The “ Boreal” (Hol- 
arctic realm) “ Province extends obliquely across the entire con- 
tinent from New England and Newfoundland to Alaska, con- 
forming in direction to the trend of the northern shores of the 
continent. It gives off three long arms or chains of islands which 
reach far south along the three great mountain systems of the 
United States, a western arm in the Cascades and Sierra 
Nevada, a central arm in the Rocky Mountains, and an eastern 
1 Biological survey of the San Francisco Momitain; N. Amer. Fauna, No. 3, 
1890, p. 24. 
