mabshJ CONCLUSION. 227 



PAET IV. 



co^cLtxsiOjsr. 



The brief review of Xorth American Dinosaurs given in the jjreced- 

 ing pages, in connection with the accompanying illustrations, will make 

 the reader acquainted with the more important type specimens of this 

 interesting group of reptiles, as now known from this continent. To 

 discover and bring together these remains, representing several hun- 

 dred individuals, from widely separated localities and various geological 

 horizons, has been a long and laborious undertaking, attended with 

 much hardship, and often with danger, but not without the pleasure 

 that exploration in new fields brings to its votaries. These researches, 

 especially in the West, have been continued by the writer more than a 

 score of years, and have led him across the Eocky Mountains a still 

 greater number of times. The field work thus prosecuted has been of 

 great service in the subsequent study of the specimens secured, espe- 

 cially iu determining the natural position in life of each animal 

 investigated. 



Iu comparing the type specimens of these various animals, one with 

 another, as they were found and as they appeared when removed from 

 the vesture of their entombment, many questions have suggested them- 

 selves that can not be answered in the present, limited paper. Resem- 

 blances and differences are striking, both in structure and form, in these 

 ancient reptiles, but the true meaning of such features is a difficult 

 problem to solve. On the interpretation of characters thus exhibited in 

 these animals depend both the laws of their classification and theories 

 of their origin. 



COMPARISON OF CHARACTERS. 



In the concluding part of the present paper a number of plates 

 (LXXYI-LXXXI) have been given with a view to illustrate especially 

 the corresponding parts of various animals of different orders, show- 

 ing the wide divergence in some points of nearly allied forms, and the 

 approach in particular features of types clearly distinct. 



In PI. LXXVI four skulls of as many typical genera of herbivorous 

 dinosaurs (Triceratops, Claosaurus, Camptosaurus, and Diplodocus) 

 are represented, with a cast of the brain cavity of each in position. 

 All are so drawn that they can be readily compared, thus exhibiting in 

 a striking manner both the diminutive size of the brain in each in pro- 

 portion to that of the skull, and also the form of the brain cavity, when 

 seen from above. In the next plate (LXXVII) the brain casts alone of 

 several dinosaurs, as seen from the side, are exhibited, and with them 

 for comparison the corresponding cast of a young alligator. The spe- 

 cial features of the dinosaur brain are well shown in these two plates. 



PL LXXTIII will make clear the wide divergence of forms of teeth 

 in four different families of predentate dinosaurs. The typical genera, 



