4 FOSSIL TURTLES OF NORTH AMERICA. 
costals as rib-heads, which articulate with the vertebral centra. At the front of the 
shell each rib-head articulates with two centra, at their junction with each other; 
more posteriorly, the rib-head falls farther and farther behind the intervertebral 
articulation. There is no movement of the rib-heads on the centra, nor of these on _ 
one another. 
The first dorsal vertebra articulates with the last cervical, permitting a very 
free motion. Its ribs are short, have no costal plate, and lie closely joined to the 
front of the rib of the first costal plate. From this it will be seen that the second 
rib has coalesct with the first costal plate, the third rib with the second costal, and 
soon. The tenth dorsal rib is short and slender; its distal end is consolidated with 
the eighth costal plate, and has the upper end of the ilium abutting against it. 
In life the bones of the carapace and the plastron are covered by a number of 
horny plates, the scutes. Where the edges of any two of these meet they impress a 
furrow in the bone, forming a sulcus. When the skeleton is prepared the scutes 
fall off, but the sulci remain to betray the number and form of the scutes. In the 
figures here shown (figs. I, 2), as in nearly all the figures of this work, the sulci are 
represented by stippled bands; the sutures between the bones, by zigzag lines. 
In fig. 1 the scutes are represented on both sides of the shell, but the symbols of 
the names are placed only on the right side. Immediately in front, at the midline, 
there is a small seute, the nuchal (nu.s). “Then comes a row of five large scutes, 
the vertebrals (J. 1, ¥. 2, etc.), each extending out beyond the neural bones. The 
sulci separating these seutes cross respectively the first, third, fifth, and eighth neu- 
rals. On each side of these vertebrals is a row of four large seutes, the costals(c.-s, 
I; eS, 2; etc.). The sulci between these descend respectively on the second, 
fourth, sixth, and eighth eostal bones. 
The borders of the carapace are invested by a series of marginal seutes, twelve 
on each side (f, 2,3, etc., on-right side). The sulci dividing these from the costal- 
scutes run along near the upper border of the peripheral bones. At the free borders 
of these peripherals the scutes turn down and appear on the under sides of the 
bones. “* fe her 
It will be observed that the scutes coincide neither in number nor position with 
the underlying bones. It is seldom that the sulci follow the sutures. ‘This matter 
will be discust hereafter. 
In fig. 2 are represented the plastral seutes, the characters indicating the names 
appearing in the right side of the drawing. A median longitudinal suleus runs 
from the front to the rear of the plastron, separating the-scutes of each pair. In 
front is a pair of gular seutes (g)s then a pair of humerals (hum); followed by the 
pectorals (pec), the abdominals (aby, the femorals (fem), and finally, by the anals 
(an). Just behind each axillary notch is an axillary seute; while just in front of 
each inguinal notch is found an inguinal scute. .“The marginal seutes-seen in this 
figure-are the same that appear in fig. I. 
We may now examine the vertebrz in front of and behind the dorsals. In all 
of the turtles there are normally 18 presacral vertebra, of which 8 belong to the 
neck. The more anterior and the more posterior cervicals are the shorter. The 
neck as a whole is about as long as the dorsal series of vertebrae. ‘The first is com- 
posite, consisting of four distinct pieces. On each side is a neural arch, aiding in 
forming the neural canal. Below, these abut on a median piece, the hypocentrum. 
These three bones unite in forming a concavity, into which fits the ball-like occipital 
condyle. Behind the arches and the hypocentrum 1s the odontord process, the proper 
centrum of the first cervical. Behind, this articulates with the centrum of the 
second cervical but does not become anchylosed with it. 
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