30)8 Jt'. S. Lull — Stajomuru.s imgulatus Marsh. 



Trachodon, in which the powerful dental battery of 2,000 

 teeth reaches its liighest perfection. 



III. Armamoit* 



The exoskeleton of Stegosminis included five types of struc- 

 ture, of which the first consists of small rounded ossicles 

 found in one instance in position beneath the throat. The 

 known armor of the back consisted of four distinct forms of 

 plates in each of which the dorso-ventral diameter is enor- 

 mously developed, and often the antero-posterior one as well, 

 instead of a normal horizontal expanse, the result beiii^ the 

 production of huge upstanding plates arranged in pairs on 

 either side of the median line of the neck, trunk, and proximal 

 part of the taij, while immense bony spines, possibly varying 

 in numbers with the different species, were borne on the distal 

 end of tlie latter. The plates are differentiated in that those 

 borne upon the neck are short in fore and aft diameter but 

 with a heavy base divided longitudinally by a depi'ession which 

 was borne astride tlie transverse processes and cervical ribs. 

 The trunk plates increase rapidly in dimensions to a maximum 

 size of 71X66™ ( = 28x26 inches) and a weight of 40 pounds 

 for the pair borne over the pelvis. Here the base was thick 

 and rugose but witliout the longitudinal cleft. These plates 

 were borne above the transverse processes and the proximal 

 portion of the ribs until the sacrum was approaclied with its 

 widely expanded neural spines to which the weiglit was trans- 

 ferred, the two rows of plates approaching each other more 

 nearly than in the more anterior region. 



This same type of plate, though with a base somewhat 

 shortened and at the same time much broader, was borne over 

 similar neural spines on the anterior third of the tail. Here 

 the character of the neural spines abruptly changes, the broad- 

 ened summits being lost and the whole spine very much 

 reduced in height. This point, which marks the beginning of 

 the aggressive, flexible portion of the tail, marks also an abrupt 

 change in the character of the plates, which are now sharp- 

 edged, pointed and bent backward, the base being embedded 

 in the muscles between the neural spines and the vertebral 

 centra. The sharp-edged plates, of which there are three 

 pairs, are followed by four pairs of long spines similarly 

 attached and pointing outward and backward at a decided 

 angle. The longest of these is 25 inches and its present 

 weight 14r^ lbs. 



The character of the surface of the thin expansion of the 

 plates, which doubtless represents the hypertrophied median 



*Lull, this Journal (4), xxix, pp. 201-210, figs. 1-11, 1910. 



