206 E. S. Lull — Armor of Stegosaurus. 



Evidence for pairing of the entire series is shown in two 

 specimens preserved in the IT. S. National Museum, in one of 

 which the plates alone are represented while in the other they 

 are actually in association with the underlying bones. These 

 plates if placed consecutively would measure twice the length 

 of the neck and back, the proportion being 16 to 8 feet. This 

 evidence, together with the fact that each individual plate 

 as shown above is in itself not symmetrical, indicates that the 

 plates were not median but lateral structures and were arranged 

 in at least two rows. 



The first restoration showing the plates in two rows is given 

 in a drawing made by Charles R. Knight under the direction 

 of F. A. Lucas and published by the latter first in his book 

 "Animals of the Past," New York, 1901, fig. 24, and again in 

 the Smithsonian Report for 1901, plate iv. Later, under Mr. 

 Lucas's direction, a model was made by Mr. Knight in which 

 the number of caudal spines was reduced to two pairs and the 

 plates were placed in such a way as to alternate along the back. 

 The reasons given for this arrangement were two-fold : first, 

 that the plates did actually alternate as they lay embedded in 

 the rock, and second, that no two of them were precisely 

 similar in exact shape or dimensions. Against the argument 

 that no known reptile has alternating dermal elements was 

 urged the apparent fact that this did not render it an impossi- 

 bility in Stegosaurus. It seems to me, however, that the posi- 

 tion of the plates in the rock is hardly conclusive, for the series 

 of one side might easily have shifted forward or backward 

 slightly during maceration or in the subsequent movement of 

 the rocks, as an oblique crushing of fossil bones is a very 

 familiar phenomenon. 



The slight disparity of size and shape in the two plates of a 

 pair is not surprising when one considers that the entire 

 hypertrophy of the plate is in a sense abnormal and is com- 

 parable to the growth of the antlers of deer of which those 

 borne by an individual are rarely if ever precisely similar in 

 size, weight, form, or even in number of points. I should con- 

 sider a precise matching of the stegosaur plates remarkable 

 rather than the reverse. The fact that in no other reptile the 

 lateral dermal elements alternate seems too weighty an argu- 

 ment to be lightly dismissed. 



The evidence in favor of an erect rather than a procumbent 

 or imbricating position is the morphology of the plate itself, 

 as described above, and the fact that in the crocodile and gavial 

 one can witness the actual hypertrophy of the median keel in 

 the two rows of scutes, which finally merge into one along the 

 mid-dorsal line of the distal half of the tail. The elevation of 

 the keel becomes more and more pronounced beginning with 



