G48 GEOLOGICAL SURVEY OF THE TERRITORIES. 



HO.MO. KUMIXASTIA. EqUUS. 



Hy^na. . SiMiUJE. Tragulus. Hipparion. Elefhas. 



? PiNNIl'EDIA. FELIS. 



Ganis. 



Hv<eriiodon. 



Uusus. Nasua. 



Cebus. Anoplotbcrium. Anchitherium. Dinotherium 



Hapale. Omxivoua. Orohippus. Eofcasileus. 



Lemuk. 



Synoplathorium. Cercoleptes. Tomitiieiium. 



/ 

 Palaeo.syops. Bathmodon. 



? Oligotomus. 



: Note. — Recent genera in shall capitals ; Miocene in italics,- and Eocene in ronian. 



Oh the plnjlogeny of the (jenerd of Tcstudinata. — The extinct tortoises 

 of the Cretaceous and Eocene throw considerable light on the probable 

 origin of various existing genera,* and while much remains obi'cure, 

 the following observations may be derived from the study of the forms 

 in question : 



The order makes its appearance in the Triassic period, for I am as- 

 sured by Dr. F. Endlich, of Reading, Pennsylvania, that the species 

 obtained by Professor Queustedt in Wiirtembnrg belong undoubtedly 

 to the Testudinata. With their special structure we are not yet fully 

 acquainted. A number of genera appear in the Jurassic, and there is 

 a successive increase in the number of species in the Cretaceous and 

 Tertiary formations. Three structural features of importance mark the 

 earlier fornSs. First, the incomplete union and ossilication of the ele- 

 ments of the plastron and carapace; second, the reduction in size of 

 the lobes of the plastron ; third, the natatory character of the {)ha- 

 langes, by their truncation and union in a single plane. Genera, re- 

 taining some or all of the peculiarities, persist to the present day ; but 

 the ossified types, with distinct digits, are far more abundant, and are 

 comparatively rare in the })eriod of the Jura. 8i)hargis, which is with- 

 out carapace and has a greatly reduced plastron, may be regarded as 

 nearest the primitive types of the order, though it still exists. Protoa- 

 tega, of the Kansas Cretaceous, is its nearest extinct ally known. Fro- 

 tostega is sui)erior in the well-developed marginal bones, and prepares 

 the way for consideration of the various genera, with incomplete shields 

 of tlie present period (Chelone) or of the Jurassic; .the former possess- 

 ing the natatory extremities, some of the latter assuming a terrestrial 

 niodilication of limbs. Those with ambulatory limbs lead us at onee 

 to the existing Chelydra, the closing of the sternal fontanelles being ac- 

 companied by a contraction of its extent, in respect to the bridges and 

 lobes. In FrojHeura of the Cretaceous we have a state of things inter- 

 mediate between some of the Jurassic genera, as Idlochelys and Chelone. 



* See ou the Extiuct Tortoises of the Cretaceous of New Jersey ; Proceedings 

 Amer. Assoc. Adv. Science; 1871, p. 344. 



