Ji. aS'. Lull — Stegosaurus imgulatus Marsh. 373 



" Animals of the Past," and again in tlie Smitlisonian Report 

 for 1901, Plate IV. The second restoration is a statuette one- 

 tenth linear dimensions. In the drawing, the plates along the 

 back were placed opposite to each other in pairs and the tail 

 bore four pairs of spines, whereas in the later model the 

 .plates alternated and the pairs of spines were reduced to two. 

 The apparent evidence for the alternation of the plates is 

 offered by a specimen in the collection of the U. S. JNational 

 Museum, in which they are preserved in the rock in the alter- 

 nating position and in the fact that those of the opposite sides 

 differ materially in shape and size. This Lucas still thinks 

 conclusive, in spite of the fact that no known reptile has alter- 

 nating dermal scutes, and that the probabilities are that the 

 one row of the plates, as preserved in the matrix, has shifted 

 forward or backward during maceration or in the subsequent 

 movement of the rocks, as an oblique crushing of fossil bones 

 is a very familiar phenomenon. The disparity in size and 

 shape of 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 comparable to the growth of the antlers of 

 dee)', notably the caribou, of which those borne by a single 

 individual are rarely if ever precisely similar in size, weight, 

 form, or even in the number of points. 



The reduction of the number of caudal spines was also due 

 to the evidence offered by two other specimens, also in the 

 U. S. National Museum, with thespines insitu. The type of 

 Stegosaurus ungidatus, however, shows four pairs and no fur- 

 ther evidence of the duplication of bone, so that it is evident 

 that they all belonged to one individual. It would seem, there- 

 fore, as though the number both of spines and plates may well 

 have been a specific character if not an individual or possibl}'' 

 sexual variation. 



During the erection of the Yale specinaen, an admirable 

 opportunity was offered for another model of which I have 

 taken advantage, this time using a scale of one-fifth to reduce 

 still further the margin of error. (Fig. 10. See Plate II.) 

 The model is based not alone upon the proportions and posi- 

 tions of the various bones as in the mounted skeleton, but also 

 upon studies of the individual bones made with a view of plot- 

 ting thereon the attachments of the various skeletal muscles 

 of the body and limbs. In this work, a small alligator was dis- 

 sected and for comparison a specimen of Sphenodon {Hatteria) 

 as well. I was also aided by the admirable work of v. Huene 

 on the musculature of Plateosaurics erlenbergiensis (Die Dino- 

 saurier der Europaischen Triasformation, Jena, 1908) as well as 

 by Haughton's work on the muscular anatomy of the crocodile. 

 One side of the model, the left (fig. 10), shows the superficial 



