STUDIES FOR STUDENTS. 
tHE DEVELORMENTD AND GEOLOGICAL REIWATIONS 
Ol ISS, WAS IRIN GBI IIGS, 
AVE EES Se 
THe Amphibia may be described as forms that live a portion 
of their lives, at least, in the water or in a condition fitted for 
an aquatic existence; the latter statement is necessary from the 
fact that some forms never live in the water, but develop 
external gills such as are found in the forms which do pass the 
larval stage as aquatic forms. All amphibians pass through a 
metamorphosis in which the external gills of the immature 
forms are lost and lungs developed. The skull presents some 
peculiarities ; the whole base of the cranial region is covered by 
a large presphenoid bone that extends far forward as well as 
backward. In the more highly developed types of the verte- 
brates this bone occupies a minor position anterior to the basi- 
occipital. In many of the more primitive forms of the amphib- 
ians the bones of the cranial region are largely cartilaginous ; 
the extremities of the lonz bones of the limbs are without the 
epiphyses, or separate ossifications, that are found in the mam-_ 
mals, and the ribs are attached each to a vertebra imsteadsor 
intervertebrally as in the higher forms. 
That the Amp/ibia were derived from the /%sces is without 
question, but from what branch of the piscine stem they were 
developed has been the subject of much discussion. Fora long 
time they were supposed to be derived from the Dzpnoans, and 
from forms that were very close to the existing genera of the 
order. This idea is still supported by Haeckel (Systematische 
Phylogenie der Wirbelthiere; Berlin, 1895). In discussing the 
question, Baur said (The Stegocephali: a phylogenetic study ; 
500 
