FOSSIL VERTEBRA TES— AMPATBIA 503 
but in most cases, on the ventral surface, at least, protected by 
the development of small dermal ossicles of bone; the legs were 
in most of the forms rather short and weak, and in some cases 
were entirely atrophied as in the recent snakes; the bones of 
the more primitive forms were quite largely cartilaginous, the 
carpus and the tarsus of some forms being almost entirely so, 
and even the long bones of the limbs and the bones of the skull 
were incompletely ossified; the skull was completely roofed 
over by dermal bones developed in the skin of the head, leaving 
only five openings in the skull, the two nostrils, the orbits, and 
the pineal foramen, the last a single opening on the upper sur- 
face of the skull. The teeth of most of the forms, and of the 
more highly developed forms of the Trias especially, are notable 
for the peculiar infolding of the enamel, giving the internal 
structure of the tooth a most complicated appearance. 
The structure of the spinal column in the various forms is of 
especial interest, as it parallels in the first stages the history of 
the spinal column of the fishes and takes the process a few steps 
farther. In all of the Ladbyrinthodontia the notochord is to a 
SLeatcimon wleEss extent persistent. In the simplest of the 
Amplibia the chord is very large and the vertebre are repre- 
sented by small plates arranged in pairs on the upper and lower 
surface of the chord; to these plates are attached the spinuous 
processes and the hemal arches that protect respectively the 
spinal chord and the nutrient arteries (in the amphibians the 
hemal arches are complete only in the caudal region). The 
next stage in the development of the column is the union of the 
plates, on the upper and the lower side of the chord, to form a 
complete cylinder that incloses the chord. The cylinder is of 
equal bore throughout and does not constrict the chord or tend 
to divide it up into segments. These two conditions are found 
only in the most simple of the amphibians and are well illus- 
trated in the immature and mature forms of the Branchiosauria 
(Fig. 1,4). The next step is the thickening of the walls of the 
middle portion of the cylinders that surround the notochord so 
that the bore is very small at the center, while at the anterior 
