Stegosaunm prisms, op. nov. 151 



a longitudinal cleft occurs between these two bones. Compared with 

 the ischium of 0. armatus (Fig. 8a) it may be seen that the distal 

 half, though it has on its upper margin the notch characteristic of 

 Stegosaurus, is much more slender in St. priscus, but the flat, desk- 

 like shape is the same in both animals. If we compare this type of 

 ischium with the same bone in St. ungulatus or 0. Lennieri we find 

 a fundamental difference, for in these two genera the ischium is not 

 flat but twisted, and the superior margin of the ischium curves in 

 such a manner as to meet in the median line at the proximal part of 

 the rather long ischiadic symphysis, so that a great part of the 

 ischium is thus modified into a horizontal plate that overlies the 

 pubis. I am at a loss how to explain this difference in two forms so 

 closely allied, for it seems difficult to explain it simply through post- 

 mortem pressure. The longitudinal cleft between the pubis and the 

 ischium, which is present in both species of Omosaurus and our 

 Stegosaurian, is a character found in all primitive Ornithopoda ; the 

 closing of this cleft observable in St. ungulatus must therefore be 

 regarded as a mark of specialization. 



To facilitate a comparison of the ischia of 0. armatus and St. priscus 

 it has been thought advisable to figure these bones of both species 

 near each other, and this seems all the more necessary because the 

 pelvis of 0. armatus has never yet been figured in the proper position. 



Dermal ^Armour. — Associated with the bones just described several 

 badly crushed pieces of dermal armour were discovered. Both spines 

 and plates are present. 



The plates are somewhat asymmetrical. Towards their base they 

 are rounded and equally rugose on the exterior and interior margin, 

 thus proving that they rose nearly vertically out of the skin ; since, 

 however, they are all more or less in a fragmentary condition, it 

 seems enough to publish only a diagram of the transverse section 

 (Fig. 9a) to show how this kind of plate would be inserted in the 

 tissue of the body. 



Though as badly preserved as the plates, the two spines found 

 among the material show such peculiar features as to justify a more 

 detailed description. The fragments are nearly similar, and include 

 in both cases the base of a spine and its attachment-surface. The 

 height of the specimen figured is 29 - 5 cm., its breadth 10 cm., its 

 thickness at the summit 2- 5 cm. 



Of the upper part probably three-fourths or more is missing, 

 but the base is entire and scarcely deformed by crushing. The piece 

 shows an anterior and a posterior ridge, a more concave outer and 

 a flatter inner surface, the latter being divided into a superior and an 

 inferior half. On the outer side (Fig. 9b) at the base only a narrow 

 rugose margin is observable; on the inner side, however, the rugose 

 area reaches much higher (Fig. 9c) and occupies nearly half of the 

 fragment, making an obtuse angle with the upper half of the spine. 

 The great extent of the rugosity on the inner surface and the flatness 

 of the surface over which the rugosities extend prove that while the 

 exterior part of the bone was only slightly embedded in soft tissue, 

 the interior side continued to adhere to the skin up to a much higher 

 point ; hence we may conclude that this spine rose, as indicated in 



