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 

 so on. 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. I 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 scute, the nuchal (nu. s). Then comes a row of five large scutes, 

 the vertebrals (v. i, v. 2, etc.), each extending out beyond the neural bones. The 

 sulci separating these scutes cross respectively the first, third, fifth, and eighth neu- 

 rals. On each side of these vertebrals is a row of four large scutes, the costals (c. s, 

 i; c. s, 2; etc.). The sulci between these descend respectively on the second, 

 fourth, sixth, and eighth costal bones. 



The borders of the carapace are invested by a series of marginal scutes, twelve 

 on each side (i, 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. 



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 scutes, the characters indicating the names 

 appearing in the right side of the drawing. A median longitudinal sulcus runs 

 from the front to the rear of the plastron, separating the scutes of each pair. In 

 front is a pair of gular scutes (g); then a pair of humerals (hum); followed by the 

 pectorals (pec), the abdominals (ah), the femorals (fern), and finally, by the anals 

 (an). Just behind each axillary notch is an axillary scute; while just in front of 

 each inguinal notch is found an inguinal scute. The marginal scutes seen in this 

 figure are the same that appear in fig. i. 



We may now examine the vertebrae in front of and behind the dorsals. In all 

 of the turtles there are normally 18 presacral vertebrae, 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 is the odontoid 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. 



