TESTUDINIDA:. 369 



latter family. The ends to be subserved by this convexity may be various. We may suppose 

 that it would be much more difficult for a carnivorous animal to effect an entrance into such 

 a shell than into one deprest and whose borders may be spanned by the jaws of the enemy. 

 Again, the vaulted shell provides greater space for the lungs; and the investigations credited 

 by Hoffmann (Brown's Klass. and Ordn.: Schildkr., p. 344) to Dr. Joseph Jones but really 

 made by Louis Agassiz (Contrib. Nat. Hist. U. S., vol. i, p. 283) show that the capacity of the 

 lungs of Gopherus polyphemus is much greater than that of Emydidae having an equal length 

 of shell, except Terrapene, which has similar habits of life. Even the earlier known forms of 

 the Testudimdae, as Hadnanus and Stylemys, have the shell high and vaulted; and this fact 

 suggests that the deprest form of some existing species may be due to adaptations to recent 

 conditions. The flattened carapace of Gopherus polyphemus, for instance, may have relation 

 to the burrowing habits of the species. 



Among the Emydidae the neural bones are usually hexagonal, with the broader end of the 

 bone in front. Such, too, is the case with some of the Testudinidae. In Stylemys nebrascensis 

 we find this condition prevailing; but occasionally a specimen is found in which the second 

 one is octagonal. In S. conspecta and S. capax, of the John Day beds, there is further advance- 

 ment the second is octagonal, while the third is tetragonal. Too much weight ought not to 

 be attacht to single specimens having modifications of this sort; as is illustrated by the fact 

 that in the American Museum of Natural History there is a specimen of Chelydra serpentina 

 in which the second, fourth, and fifth neurals are octagonal, while the third is tetragonal. 

 Nevertheless, among the Testudinidae a high degree of differentiation of the neurals is the rule. 

 When we come to the genus Testudo itself we find the alternation of octagonal with tetragonal 

 neurals the prevailing arrangement. In Kinyxis, according to Boulenger's figure (Cat. 

 Chelonians, fig. 41), the neurals are hexagonal with the broad end behind, a form the reverse 

 of that common among the Emydidae. In Homopus the emydoid condition prevails, except 

 that the third is tetragonal. 



The costals have suffered remarkable modifications among the Testudinidae. In most 

 other turtles these plates, from the second to the sixth inclusive, have the proximal and the 

 distal ends of each costal of nearly the same width. In most species of Testudo and in Gopherus 

 the proximal ends of the second, fourth, and sixth are much narrowed, while the distal ends 

 are greatly widened. On the other hand, the proximal ends of the third and fifth are broad, 

 while the distal ends are narrow. The proximal ends of costals 3 and 4 are in contact with 3 

 neurals each. The 5 costals concerned appear thus to be dovetailed with one another. The 

 mechanical advantages of this arrangement have been discust by A. Bienz (Rev. suisse 

 Zool., etc., in, 1895, p. 99). These modifications of the costals are not conspicuous in Had- 

 rianus and in Stylemys nebrascensis, but they become very manifest in the species of Stylemys 

 from the John Day beds. In Testudo brontops of the lowest Ohgocene, the neurals display a 

 high grade of differentiation, and, in harmony with this, the upper ends of the costals are modi- 

 fied so as to unite alternately with I and with 3 neurals; but there is no great difference in the 

 widths of the upper and lower ends of each costal. 



In most turtles the position of the rib is very distinctly shown on the visceral side of each 

 costal plate; and the heads of the ribs are broad and thick. These heads join their respective 

 vertebral centra at the anterior ends of the latter, and at least the anterior ones come into 

 contact also with the next centrum in advance. In the Testudinidae the ribs show only faintly 

 on the under side of the costals and the rib-heads are usually greatly reduced. In a specimen 

 of Testudo radiata from Madagascar, having the shell 305 mm. long, the rib-heads of the 

 second to the sixth costal plates may be 30 mm. long; but they are very slender blades 5 mm. 

 thick horizontally and 2 mm. high, expanding somewhat toward the proximal ends. In 

 Gopherus only the merest traces of the rib-heads remain of those belonging to the costals in 

 front of the seventh; and these vestiges adhere to the centra and to the neural. In T. tabulata 

 the rib-heads are all much reduced. Many of these are wholly free from the costal plates and 

 have become co-ossified with the under sides of the neural bones. Intermediate stages are to 

 be seen in this species. This condition illustrates well the manner in which the ribs, probably 

 free from the costals in the most primitive turtles, have become consolidated with them. 



When we examine Hadnanus we find that the rib-heads were somewhat reduced, but still 

 of considerable size. In Stylemys they are still more reduced. A careful examination of a 

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