marsh.] DISTRIBUTION OF STEGOSAURIA. 195 



the skull, so that its position in life may be regarded as definitely set- 

 tled. The series of vertical rdates which extended above the neck, 

 along the back, and over two-thirds of the tail- is a most remarkable 

 feature, which could not have been auticipated and would hardly have 

 been credited had not the plates themselves been found in position. 

 The four pairs of massive spines characteristic of the present species, 

 which were situated above the lower third of the tail, are apparently 

 the only part of this peculiar armor used for offense. In addition to 

 the portions of armor above mentioned there was a pair of small plates 

 just behind the skull, which served to protect this part of the neck. 

 There were also, in the present species, four flat spines, which were 

 probably in place below the tail, but as their position is somewhat in 

 doubt they are not represented in the present restoration. 



All these plates and spines, massive and powerful as they now are, 

 were in life protected by a thick, horny covering, which must have 

 greatly increased their size and weight. This covering is clearly indi- 

 cated by the vascular grooves and impressions which mark the surface 

 of both plates and spines, except their bases, which were evidently 

 implanted in the thick skin. 



The peculiar group of extinct reptiles named by the writer the Steg- 

 osauria, of which a typical example is represented in the present resto- 

 ration, is now nearly as well known as any other dinosaurs. They are 

 evidently a highly si^ecialized suborder of the Predentata, which have 

 the Ornithopoda as their most characteristic members, and all doubt- 

 less had a common ancestry. 



Another highly specialized branch of the same great order is seen 

 in the gigantic Ceratopsia of the Cretaceous, which the writer haa 

 recently investigated and made known. The skeleton of the latter 

 group presents many interesting points of resemblance to that of 

 the Stegosauria, which can hardly be the result of adaptation alone, 

 but the wide difference in the skull and in some parts of the skeleton 

 indicates that their affinities are remote. A comparison of the present 

 restoration with that of Triceratops on PI. LXXI will make the con- 

 trast between the two forms clearly evident. 



DISTRIBUTION OF STEGOSAURIA. 



All the typical members of the Stegosauria are from the Jurassic 

 formation, and the type specimen used in the present restoration was 

 found in Wyoming, in the Atlantosaurus beds of the upper Jurassic. 

 Diracodon, a genus nearly allied to Stegosaurus, occurs in the same 

 horizon. Palaeoscincus Leidy, 1S56, from the Cretaceous, and Pricono- 

 don of the writer, 1888, from the Potomac formation, are perhaps allied 

 forms of the Stegosauria, but until additional remains are found their 

 exact affinities can not be determined. Apparently the oldest known 

 member of this group in America is the Dystrophseus Cope, 1877, from 

 the Triassic of Utah. 



