212 DINOSAURS OF NORTH AMERICA. 



The teeth in this family are entirely confined to the maxillary and 

 dentary bones. The rostral bone, the premaxillaries, and the pre- 

 dentary are entirely edentulous. 



CERVICAL AND DORSAL VERTEBR.E. 



The atlas and axis of Trieeratops are coossified with each other, and 

 at least one other vertebra is firmly united with them. These form a 

 solid mass, well adapted to support the enormous head (PI. LXIV, 

 fig. 1). The cup for the occipital condyle is nearly round and very 

 deep. The rib of the second vertebra is coossified with it, but the 

 third is usually free. The centrum of the fourth vertebra is free, and 

 the remaining cervicals are of the same general form, all having their 

 articular faces nearly flat. 



The anterior dorsal vertebra' have very short centra, with flat artic- 

 ular ends, and resemble somewhat those of Stegosaurus, especially in 

 the neural arch. This is shown in PI. LXIV, tigs. 3 and 4. 



The posterior trunk vertebrae have also short, flat centra, but the 

 diapophyses have faces for both the head and tubercle of the ribs, as 

 in crocodiles, a feature but recently seen in dinosaurs. 



THE SACRUM. 



The sacrum was strengthened by union of several vertebras, ten 

 being coossified in one specimen of Trieeratops (PL LXV). The middle 

 or true sacral vertebra} have double transverse processes, diapophyses 

 being present and aiding in supporting the ilium. This character has 

 been seen hitherto in the Dinosauria only in Geratosaurus and some 

 other Theropoda. 



The main support of the pelvis was borne by four vertebra-, which 

 evidently constituted the original sacrum. 1 n front of these, two others 

 have only simple processes, and apparently were once dorsals or lum- 

 bars. Three vertebrae next behind the true sacrum have also single 

 processes, and the fourth, or last of the series, has the rib process weak, 

 and not reaching the ilium (PL LXV). Seen from the side the sacrum 

 is much arched upward, and the neural spines of the true sacrum are 

 firmly coossified. In the median region the sacral vertebra 1 have their 

 centra much compressed, but the last of the series are widely expanded 

 transversely. The whole appearance of the sacrum is remarkably avian. 

 The neural canal of the sacral vertebrae has no special enlargement, 

 thus differing widely from that in Stegosaurus. 



THE CAUDAL VERTEBR.E. 



The caudal vertebras are short and the tail was of moderate length. 

 The first caudal has the anterior face of the centrum concave vertically, 

 but flat transversely, and a short, massive neural spine with expanded 

 summit (PL LXIV, tigs. 5-7). In the median caudals the centra have 

 biconcave articular faces and weak neural spines. The distal caudals 

 are longer than wide, with the ends concave and nearly round. 



