164 GEOLOGICAL DISTEIBUTIOX. 



birds, having its origin in the dinosaurian reptiles, and in a form 

 possibly not distantly removed from the Jurassic Compsognathus. 

 The more important Jurassic genera of Pterosauria are Pterodacty- 

 lus, Rhamphorhynchus, and Dimorphodon, which differ from each 

 other mainly in the character of the dentition and in the relative 

 development of the tail. 



The deposits of this age have yielded, in addition to the reptilian 

 forms mentioned, the remains of true lizards, crocodiles, and turtles. 

 The first are but sparingly represented, and in the main, or wholly, 

 belong to the acrodont division (Geosam-us, Acrosaurus, Homoeo- 

 saurus, the last closely related to the modern Lacerta). The croco- 

 diles, with some partial exceptions (Streptos^wndylus, Theriosuchus), 

 belong to the primitive amphicoelous division (Teleosaurus, Mystrio- 

 saurus), or those in which there is a retention of the ichthyic char- 

 acter of bi-concave vertebrse, as distinguished from the more modern 

 forms dating from the Cretaceous period, with procoelous (concavo- 

 convex) vertebrae. The turtles, which appear here for the first time, 

 exhibit a remarkable differentiation, and in their diverse forms 

 comprise representatives of several of the more important modern 

 groups, as the Chelydae (Plesiochelys, Craspedochelys, Pleuroster- 

 non), Emydae (Thalassemys), and Chelydridse. The recent genus 

 Chelone is found in the Purbeck beds. 



The most interesting addition to the fauna of this period is fur- 

 nished by the bird remains, whose earliest unequivocal traces are 

 found in the famous Archoeopteryx of the Solenhofen slates of 

 South Germany (Bavaria), and in Laopteryx, from the deposits of 

 Wyoming Territory — the last a bird probably of about the stature 

 of a crane, but with uncertain afiinities. The Archsoopteryx, which 

 is known by two more or less well - preserved specimens, and a 

 feather pertaining to a third individual, combines in a most ex- 

 traordinary manner what have generally been considered distinc- 

 tively avian and reptilian characters, and, indeed, the animal may 

 be regarded as a type intermediate between the two classes. Thus, 

 in many points of structure of the skull and trunk, no less than in 

 the structure of the tail, which was greatly elongated and made up 

 of numerous distinct vertebrae, it is decidedly reptilian, and this 

 relationship to the class of animals next lower in the scale of organ- 

 isation is borne out by the discovery, recently made by Professor 

 Dames, that the extremities of both jaws were provided with a num- 



