3790 FOSSIL TURTLES OF NORTH AMERICA. 
series of fossil turtles of the various geological periods would doubtless show a gradual reduction 
of the size of the rib-heads in this family. 
The distal ends of the costal ribs of the Testudinida have become reduced in an interesting 
way. Inthe great majority of turtles the distal ends of these ribs project more or less bey ond 
the margins of the costal plates, and the projecting ends are usually received by pits in the 
adjacent. peripherals. In most of the Testudinide the ends of the ribs have disappeared. 
In Gopherus we find that, instead of the rib entering the peripheral, a broad process of the 
peripheral rises and enters a notch on the inside of the costal plate, an interesting reversal of 
conditions. The relations of the ribs and peripherals in 7. radiata are very similar. In T. 
tabulata no conspicuous process is found on either the costal plate or on the peripheral. In 
Hadrianus, as in Gopherus and most species of Testudo, a process of the peripheral enters an 
excavation in the inner surface of the costal. On the other hand, in Stylemys the extremity 
of the rib fits into a pit in a peripheral, just as in the Emydide. 
In typical Testudinide of to-day the peripherals and contiguous costals are articulated, 
not only by the processes just described, but also by digitating sutures, and these are usually 
better developt than in the Emydide. In 7. radiata long slivers of bone arising from the 
peripherals interdigitate with shorter processes from the costals. On the other hand, 1 
Stylemys and Hadrianus there appear to have been no such digitations; but the rounded 
border of each peripheral joins the similar border of the neighboring costal. 
In the majority of the Testudinida the epiplastral bones are thickened for some distance 
on each side of their symphysis. The thickening takes place on the upper side of the bones. 
At the hinder border of the thickening, on each side of the midline, the elevation drops off 
suddenly to the level of the entoplastron. In many species the ledge thus formed and looking 
backward is excavated more or less deeply. Usually the thickened epiplastra project more or 
less beyond the general curve of the rest of the anterior lobe, and there is thus formed a con- 
spicuous epiplastral lip. This lip presents various forms in the different species and, tho 
subject to variations, it furnishes valuable specific characters. The lip is conspicuously 
developt in Hadrianus, as in most species of Testudo. In Stylemys the front of the epiplastra 
is thickened, but it does not project beyond the curvature of the rest of the lobe. 
The hinder end of the plastron is usually, but not always, notcht. 
At the anterior ends of the bridges the hyoplastron of each side sends upward a process, 
the axillary buttress, which artic tilates with the first costal plate. Usually this buttress does not 
pass above the lower border of the costal. At the hinder end of the bridge the hypoplastron 
sends up an inguinal buttress, which also rarely rises above the lower bories of the costals 
involved. In Gopherus and the species of Te studo already mentioned this buttress 1s articu- 
lated with the middle of the lower border of the sixth costal; but it appears that in some 
other living genera of the family, possibly in some other species of Testudo, the buttress is 
attacht somewhat further forward. In Hadrianus corsoni the buttress ascends a short distance 
on the inner side of the sixth costal. In Stylemys nebrascensts it is articulated between the fifth 
and sixth costals, rising a short distance above their lower borders. It is evident that there has 
been a tendency among the Testudinidz to push the buttress backward, and thus increase 
the width of the bridge. The purpose has apparently been to diminish as much as possible 
the hinder opening of the shell. 
The shoulder-girdle of the Testudinide appears to be distinguisht from that of the 
Emydidz in two respects: The coracoid is greatly expanded at its inner, or mesial border, and 
the procoracoid process makes an obtuse angle with the body of the scapula. Probably in all 
Emydide the coracoid is only slightly expanded at the mesial border and the procoracoid 
process makes an acute angle with the body of the bone. 
The humerus, in probably all members of the family, is characterized by having the radial 
and ulnar processes both twisted toward the ventral side of the bone, thus making the angle 
between them small. Usually the humerus is considerably bent, but often no more so than in 
some species of Emydide. ; ; 
In most species of the family there are 5 digits belonging to each forelimb; but occasionally 
there are only 4. No digit possesses more than 2 phalanges. How this reduction has been 
brought about is an interesting question. A similar reduction of the phalanges to two in each 
digit occurs in the foretoot of Terrapene as observed by Dr. George Baur (Zool. Anzeiger, Xv, 
1892, p. 159), 
