340 



AMERICAN TESTUDINATA. 



Part II. 



opportunities of examining these Turtles. Tlie united Chelyoidaj and Hydraspides 

 form simply a section of tlie family of Elodites in the classification of Dmneril and 



inys, Plii-ynops, and Hydraspis, agree in having no 

 temporal arch, wliile the parietals are broad, long, 

 and flat, and the parietal arch is very narrow and far 

 backward. The type of Hydromedusa and Chelo- 

 dina, which may also constitute a distinct family, 

 differs from the genuine Hydraspides in its parietals, 

 that are gradually narrowing backward to form a 

 ridge with the upper occipital, carrying the parietal 

 arch even further backward than in the Hydraspides ; 

 as in these, the temporal arch is also wanting. The 

 Podocnemides present still more striking peculiarities. 

 As in the marine Chelonioidie, the parietal and tem- 

 poral arches are united to form a broad roof over the 

 temporal region. This is the only group of Testudi- 

 nata in which the peculiarities of the skull of Che- 

 lonii and Amyda; are intimately combined. On this 

 account, I expect that the Podocnemides will be found 

 to agree much more closely, in those structural pecu- 

 liarities which constitute family characters, with the 

 earlier representatives of this order in past geological 

 ages, than with any other type. It is deeply to 

 be regretted, therefoi'e, that the beautiful series of 

 fossil Turtles found by Hugi in the Jurassic limestone 

 of Solothurn, in Switzerland, have not yet been 

 examined and described with that minuteness which 

 would furnish the means of a direct comparison with 

 the living types ; for they exhibit, more distinctly than 

 any other fossil Turtles I have seen, a surprising 

 combination of Chelonioid and Amydoid characters. 

 This is also the case with the genera Eurysternum, 

 Miinst., and Idiochelys, Myr., described by Herm. 

 von Meyer, in Miinster's Beitriige, 1839. 



It ought also to be noticed in this connection, 

 that the oldest fossil species, referred to the family of 

 Chelouioidie by Owen in his beautiful illustrations of 

 the British Reptiles, (Trans. Pahi-ont. Soc, 1851,) 

 differ in many respects from the marine Turtles, and 

 present, especially in their oval form, which is quite 

 distinct from that of the living Chelonioid^, features 

 which are characteristic of the living Emydoidas, or, 

 rather, common to all the Testudinata of the present 

 period, in the younger stages of their development. 



By its rounded form and small size, the Chelonia of 

 Glaris differs also greatly from the living Chelonioida?. 

 It certainly constitutes a distinct genus, characterized 

 by the peculiar proportions in the length of the fingers 

 of the front paddles. A knowledge of these combi- 

 nations of characters, in the earlier representatives of 

 the order, is of great importance with reference to 

 the question of their succession in former geological 

 periods, and that of their relations to the surrounding 

 mediums. Most of the oldest fossil Testudinata have 

 been referred to fresh-water types, and their occur- 

 rence in the oolitic and cretaceous rocks, with other 

 fossils evidently belonging to marine types, has led 

 to the supposition (see Pictet, Paleont., vol. i., p. 

 440) that they may have been floated into the sea 

 from the adjoining fresh waters. I hold that such an 

 assumption is not necessary. There is no closer re- 

 lation between the secondary Testudinata and the 

 living representatives of this order than between the 

 fossil Ganoids of the Jurassic and cretaceous periods 

 and the living Sauroids ; and yet it would be entirely 

 gratuitous to assume that the Jurassic and cretaceous 

 oceans were fresh-water basins, because the living 

 species of Lepidosteus and Polypterus inhabit the 

 rivers of North America and of Africa. Again : the 

 occurrence of fresh-water Turtles in the Jurassic for- 

 mation, at a period during which no Chelonioids are 

 known to have existed, would lead to the conclusion 

 that there is no relation between the gradation of 

 these animals and the order of their succession in 

 past times ; while it appears, on the contrary, that, far 

 from being genuine Emydoids, the earliest Testudi- 

 nata exhibit simultaneously synthetic and embry- 

 onic features, exactly as we have already observed in 

 man}' other types. (Comp. Part I., Sect. 24, 25, and 

 20, p. 107-118.) Now that the families of Testudi- 

 nata are better defined and more fully characterized, 

 a renewed conq)arison of the fossil and living repre- 

 sentatives of this order would add greatly to our 

 knowledge, especially if the investigation was made 

 with direct reference to the questions alluded to 

 above. The lateral movability of the neck of the 



