SZTUDIES FOR STUDENTS. 
THE DEVELOPMENT AND GEOLOGICAL RELATIONS 
OF THE VERTEBRATES: 
Ill. REPTILIA—(Continued). 
ICTHYOSAURIA. 
The Lcthyosauria or Icthyopterygia were large marine reptiles 
fish-like in their external appearance, with the body and limbs 
modified to serve the purposes of the animal in its aquatic life. 
The head terminated in front in a long and powerful snout armed 
in most cases with sharp conical teeth set in a single common 
groove which bordered the jaws. The skull was flattened above 
and the eyes were large and protected by a ring of bony plates, 
the sclerotic plates. The body was naked or covered with fine 
scales that are never preserved in the fossil forms. The ver- 
tebre of the tail were abruptly bent downward at the extremity. 
This is very peculiar because in most fossil forms in which the 
bending of the tail occurs, the bend is upward. The external 
form of the tail was as in the modern fishes. There was a dorsal 
finas in the fishes and the limbs were converted into paddles 
with a large number of phalanges. 
The Jcthyosauria were long considered as the most primitive 
of the reptiles because of the general appearance of the body 
which is so fish-like and because the paddles were considered as 
the direct modification of the fish fin. The bones of the upper 
arm and leg are so short and the carpal and the tarsal bones are 
so simple, being mere disks of bone, that they were regarded 
as the first steps from the basal segments of the fin, while the 
numerous phalanges were regarded as the segments of the fin 
rays. Now, however, there is every reason to regard the 
Icthyosauria as the specialized descendants of land-living forms. 
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