Chap. I. 



FOSSIL TURTLES. 



307 



The classes adopted in this table are circumscribed according to the principles 

 discussed in the first part of this work.* I have nothing special to add with 

 reference to the classes of Radiata, MoUusks, and Articulata; but it may be proper 

 to state here, that the order of appearance of the classes of Vertebrata makes in 

 favor of the subdivision of the Fishes into four classes. The Selachians, in par- 

 ticular, diifer so completely from the ordinary Fishes, that it is surprising they 

 have not long ago been considered as a distinct class.'^ In a palteontological 

 point of view, the early appearance of the Selachians has a deep meaning, when 

 we consider how extensively the characters of the higher classes of Vertebrata 

 (such as their few large eggs, which recall the true Reptiles and the Birds, and 

 the placental connection of the embryo of some of their species, which recalls 

 the Mammalia) are blended in their structure with embryonic features, (such as 

 their cartilaginous .skeleton and their branchial fissures,) whilst the Myzontes are 

 purely embr^^onic. The Ganoids, on the other hand, stand in a special prophetic 

 relation to the Reptiles proper;^ and their extensive reduction, at the time of the 

 first appearance of the Fi.shes proper, is truly significant. 



' The period of the first appearance of genuine 

 Fishes is somewliat doubtful, and depends upon the 

 appreciation of the true relations of the Leptolepids. 

 If they are Ganoids, as I consider them, then the 

 class of Fishes proper does not appear before the 

 Cretaceous perioil. 



* This is the period of the first appearance of 

 Tesfudinata ; at a time when neither genuine Birds 

 nor genuine Mammalia existed. 



* The presence of Birds in the Triasic period 

 is only inferred from the numerous footprints found 

 in the Red sandstone of the valley of the Connec- 

 ticut, respecting the true characters of which I have 

 expressed my doubts elsewiiere. As it is now 

 known that the earliest representatives of higher 

 types often exhibit characters common to them and 

 to lower types, it seems to me probaUh; tliat the 

 first Birds were not so completely different from 

 the other Vertebrates as the Birds now living 

 are. Before the first appearance of genuine Birds, 

 there may have existed bird-like Vertebrates, com- 

 bining in their structure Reptilian and JIammalian 

 characters, as we find early Reptiles combining Fish 

 characters, and even anticipating, in some of their 

 features, ])eculiarities that are afliTwards charac- 

 teristic of Birds and of JIammalia. Tlu' foot-marks 

 of the Trias suggest such suppositions much more 



readily than the idea of a very close affinity to real 

 Birds. For more details upon these tracks, see 

 Hitchcock, (Ed.,) An Attempt to Discriminate and 

 Describe the Animals that made the Fossil Foot- 

 marks of the United States, etc., Mem. Amer. Acad. 

 1848, vol. iii. p. 128, and Deane, (James.) Illustra- 

 tions of Fossil Footprints of the Valley of the Con- 

 necticut, Mem. Amer. Acad., 1849, vol. iv., p. 204. 

 No Bird remains are known from the Jura. 



' The presence, in the .Jurassic period, of remains 

 belonging apparently to the class of Mammalia, has 

 long been known. But Owen for the first time 

 set forth their true relations, in a paper published 

 in the Transactions of the Geological Society of 

 London, 2d series, vol. vi. Whether Microlestes of 

 the Trias, described by Plieninger, belongs to the 

 same type, is still questionable. If it is a Didel- 

 phian, it would carry this sub-class one period lower 

 down. It is curious, that nothing like them has thus 

 far been found in the Cretaceous formation. So 

 the age of Mammalia proper begins with the Eocene 

 period, unless some recently described Cetaceans 

 truly belong to the Cretaceous period. 



• See Part I., Ch. 2, p. 145, and Ch. 3, p. 183. 



' Aristotle alludes here and there to the Sela- 

 chians in contradistinction to the Fishes proper. 



' Comp. Part I., p. 116 to 118. 



