OSTEOLOGY OF THE AEMOEED DINOSAUMA. 103 



are borne. Sacrum of four fused vertebrae, sometimes with one or more lumbars added on in front; 

 thin neural canal enlarged to ten times the capacity of brain cavity. Anterior caudal vertebra the 

 largest in the column, and with strong chevron bones. Forelimb short and stout, ulna with large 

 olecranon process. Manus short, apparently pentadactyl. Femur large and straight, without inner 

 trochanter; tibia and fibula much shorter. Astragalus and calcaneum fused with opposing bones of the 

 cms; pes tridactyle, digit No. I rudimentary and No. V wanting. Dermal armour consisting of two 

 rows of flattened bony plates extending from the back of the head well down the tail, the largest plates 

 situated immediately over the pelvis. Four apines on the tail, and throat protected by a shield of 

 irregular ossicles. 



In 1910 * Lull contributed an article on the Armor of Stegosaurus, in which 

 he concluded that the plates were arranged in two rows, but in pairs. Later, in 

 the same year, 2 he published a picture of the first mounted skeleton of the genus 

 and a life restoration based upon the mount. The actual assembling of the bones 

 brought about several changes from the previously accepted conceptions of the 

 animal. It was also shown that the species S. duplex is not valid, and that the 

 type-specimen of that species is one of the cotypes upon which Marsh founded 

 the species S. ungulatus. 



Zittel, 3 in 1911, included all of the armored Dinosauria under the family 

 Stegosauridse and redefined the genus Stegosaurus. 



As shown by the recent study of the extensive collection of Stegosaurian 

 remains in the United States National Museum, the earlier definitions of the genus, 

 as briefly reviewed above, are in several respects in error. Such inaccuracies as 

 have been detected can be attributed in most instances to the incompleteness of 

 the material at the command of the earlier authors. 



In the light of new and better preserved specimens, the genus Stegosaurus 

 may now be characterized as follows: 



Generic characters. — Skull relatively very small and brain cavity in proportion 

 to size of the body more diminutive than in any other land vertebrate. Supraor- 

 bital region composed of two separate bones. Eamus deep and without lateral 



23 + 

 foramen. Dentition ^ . Teeth extremely small and weak. Vertebrae slightly 



amphiccelus, or with flat ends; dorsals with much elevated neural arches. Neural 

 canal in sacrum enlarged to many times the capacity of the brain. Sacrum and 

 lateral processes entirely roofed over by bone. Anterior caudal vertebra largest 

 in the column. Ulna (in adults) as long as humerus, with large olecranon process. 

 Manus short, pentadactyle. Phalangial formula of digits III and IV apparently 

 reduced. Femur long, straight, fourth trochanter vestigal, with or without lesser 

 trochanter. Tibia having greatest diameter of two ends in the same plane. Pes 

 tetradactyle, digit IV consisting of a vestigal metatarsal, digit V wanting. 

 Dermal armor consisting of two rows of erect, flattened plates extending from 

 back of head to distal fourth of tail. Plates of opposite rows alternating. Four 

 dermal spines near end of tail. 



Stegosaurus armatus is the type-species of the genus, and was founded upon a 

 specimen from the Morrison beds near Morrison, Colorado. Fourteen species have 



1 Amer. Journ. Sci., vol. 29, 1910, pp. 201-210, figs. 1-11. 



« Idem, vol. 30, 1910, pp. 361-377, figs. 1-10. 



3 Grundziige der Palaeontologie, vol. 2, 1911, pp. 293-295. 



