TRANSACTIONS OF SECTION D. 7 63 



9. On the Classification and Affinities of Dinosaurian Reptiles. 

 By Professor O. C. Marsh. 



The author presented briefly the results of a study of Dinosaurian reptiles on 

 ■which he had been engaged for several years. The complete results will be pub- 

 lished in a series of monographs now in preparation. The material on which the 

 investigation is mainly based consists of the remains of several hundred individuals 

 of this group collected in the Rocky Mountains by the author, and now preserved 

 in the Museum of Yale College. 



Other important American specimens have been examined by the author, who 

 has also studied with care the more important remains of this group in the 

 museums of Europe. The investigation is not yet completed, but the results 

 already attained seem to be of sufficient interest to present to the Association 

 at this time. 



In previous publications on this subject, the author had expressed the opinion 

 that the Dinosanria should lie regarded, not as an order, but as a sub-class, and 

 his later researches confirm this view. 



The great number of subordinate divisions in the group, and the remarkable 

 diversity among those already discovered, indicate that many new forms will yet 

 be found. Among those already known there is a much greater difference in size 

 and structure than in any other sub-class of vertebrates, with the exception of the 

 placental mammals. Compared with the Marsupials, living and extinct, the Dino- 

 sauria show an equal diversity of structure and size. 



According to present evidence, the Dinosaurs were confined entirely to the 

 Mesozoic age. They were abundant in the Jurassic, and continued in diminishing 

 numbers to the end of the Cretaceous period, when they became extinct. The 

 great variety of forms that flourished in the Triassic renders it more than probable 

 that some members of the group existed in the Permian period, and their remains 

 may be brought to light at any time. 



The Triassic Dinosaurs, although very numerous, are known to-day mainly 

 from footprints and fragmentary osseous remains ; hence, many of the forms de- 

 scribed cannot at present be referred to their appropriate divisions in the 

 group. 



Prom the Jurassic, however, during which period Dinosaurian reptiles reached 

 their zenith in size and numbers, representatives of no less than four well-marked 

 orders are now so well known that different families and genera can be very 

 accurately determined, and almost the entire osseous structure of typical examples, 

 at least, can be made out with certainty. 



Comparatively little is yet known of Cretaceous Dinosaurs, although many have 

 been described from incomplete specimens. All these appear to have been of large 

 size, but much inferior in this respect to the gigantic forms of the previous period. 

 The remains best preserved show that, before extinction, some members of the 

 group became quite highly specialised. 



regarding the Dinosaurs as a sub-class of the Hejitilia, the forms best known 

 at present may be classified as follows : — 



Sub-Class Dinosatjeia. 



Premaxillary bones separate ; upper and lower temporal arches ; rami of lower 

 jaw united in front by cartilage only ; no teeth on palate. Neural arches of verte- 

 brae united to centra by suture ; sacral vertebra cobssified. Cervical and thoracic 

 ribs double-headed. Ilium prolonged in front of acetabulum ; acetabulum formed 

 in part by pubes ; ischia meet distally on median line. Pore and hind limbs present, 

 the latter ambulatory and larger than those in front ; head of femur at right angles 

 to condyles ; tibia with procnemial crest ; fibula complete. First row of tarsals 

 composed of astragalus and calcaneum only, which together form the upper portion 

 of ankle joint. 



I. Order Satjkopoda (Lizard-foot). Herbivorous. Premaxillary bones with 

 teeth. Large antorbital opening. Anterior nares at apex of skull. Post-occipital 

 bones. Anterior vertebrae opisthoccelian ; cervical ribs cobssified with vertebrae ; 



