REPTILIA, BATRACHIA, AND PISCES. lil 
the Nearctic and Neotropical regions; only a small proportion of the tropical genera 
have gained a footing on the plateau, and but a few undoubted northerners have 
spread far to the south beyond it. 
The Urodela are almost exclusively holarctic and aquatic; except the monotypic 
Thorius, known only from the mountains of Orizaba and Oaxaca, but two genera, Ambly- 
stoma and Spelerpes, extend southwards into Mexico and Central America. Each of 
these genera has several species in the United States, and whilst Amdlystoma has one 
in the mountains of Siam, Spelerpes includes one from Sardinia and Italy. These 
isolated species suggest that both genera had formerly a wider and a more northerly 
range, and that only one Old World species of each managed to survive the glacial 
epoch by migrating southwards. In America Amblystoma extends on to the Mexican 
plateau and the mountains that fringe it, but Spelerpes includes about seven species 
from the United States, fifteen from the mountains of southern Mexico and Central 
America, three from the Andes of Peru and Colombia, and one from Haiti. It seems 
evident that this genus began to spread southward as soon as the elevation of Central 
America enabled it to find a congenial temperature at high altitudes. 
The Cecilians are vermiform, apodal, burrowing animals that inhabit the Neo- 
tropical, Ethiopian, and Indian regions. There are a few Central American species, 
and one of these, Dermophis mexicanus, ranges north to southern Vera Cruz. Nothing 
is known as to the past history of this group, and they may or may not be ancient 
inhabitants of South America. 
The Anura, or Frogs and Toads, may be aquatic, terrestrial, arboreal, or fossorial. 
Some genera and species have an extremely wide range, and, although it is evident 
that the sea forms an impassable barrier for the majority, it seems probable that 
accidental transmission has played a part in the dispersal of genera such as Rana 
and [Hyla. 
The two main divisions of the Anura—Firmisternia and Arcifera—may or may 
not be natural, but the further classification into families is unsatisfactory; the 
Cystignathide, for example, are heterogeneous, as the Australian genera are very 
distinct from those inhabiting South America, the latter appearing to form a natural 
group. ‘The Bufonide also may be an artificial assemblage. 
To the Firmisternia belong the Ranide and Engystomatide, abundant in South 
America, Africa, India, and ranging eastwards to New Guinea and beyond. There 
are several endemic South American genera, and two or three with Central American 
species also. Hngystoma extends from Paraguay to Texas and Florida. Hypopachus 
has one species ranging from Brazil to Costa Rica, another from Ecuador to 
Guatemala, and a third from Western Mexico to Texas. The only other genus of 
the group found in North America is the nearly cosmopolitan Rana, which extends 
