GENERAL CHARACTERS. 89 1 



in the case of very large genera, where the species differ consider- 

 ably in essential characters, attention is in some instances directed 

 to the more important specific types. The student must not, more- 

 over, expect to find that every known genus of fossil Vertebrates, or 

 even every family, is mentioned in the following chapters, of which 

 the object is to enable him to gain a fair general knowledge of the 

 past history, distribution, and leading structural features of the best 

 known groups of the various classes. 



Commencing with the outer skeleton or exoskeleton it may be 

 observed that, as a rule, the palaeontologist has but little to do with 

 structures developed in the epidermis, or layer overlying the true 

 skin or dermis, since these generally perish during the process of 

 petrifaction. The scales of Lizards belong, however, to this layer, 

 and their impressions are in some instances preserved • while in the 

 Chelonia the boundaries of the horny shields, covering the bony shell, 

 are in some instances the most important characters by which fossil 

 specimens can be determined. Occasionally, again, as in the Solen- 

 hofen Archceopteryx, the feathers of Birds have left their impression 

 in rocks of which the material is of a fine-grained structure ; while 

 still more rarely, as in the bony covering of some Glyptodonts, pits 

 from which hairs or bristles once grew are preserved in a fossil 

 state. 



The dermal exoskeletal structures are of considerably more im- 

 portance from a palaeontological point of view ; and it should be 

 observed that in nearly all the classes there appears to have been a 

 gradual tendency to the disappearance of the bony elements of this 

 skeleton in the higher forms, this being especially marked in the 

 case of Fishes. In the latter class the scales of all types, of which 

 fuller mention will be made in the sequel, belong to the dermis, 

 and in some forms there may be a complete dermal armour formed 

 of imbricating scutes, as in Callichthys among the Siluroids, or, as in 

 the Coffer-fishes (Ostracion), consisting of calcified scutes with their 

 edges in apposition. Apparently the most primitive type of dermal 

 armour in this class consists of small denticules supported on bony 

 plates, and it has been found that such denticules are absolutely 

 homologous with true teeth, of which we shall speak presently. 

 The hony pin-rays 1 and pin-spines of Fishes are another development 

 of the dermal skeleton, which will be noticed under the head of 

 that class ; although it may be observed here that many of the 

 latter are closely allied in structure to teeth. Again, the dermal 

 skeleton in many extinct Amphibians and Reptiles takes the form 

 of a more or less complete armour, either on the ventral or the 

 dorsal, and not unfrequently on both aspects of the body, con- 



1 These dermal fin-rays must not be confounded with the radial cartilages of 

 the endoskeleton. 



