Stegosaurus jjriscus, up. not. Ill 



marked in the fossil than it may have "been in the living animal. 

 The anterior and inferior margins of the centrum show strong- 

 rugosities. On the posterior margin two hypapophysial knobs are 

 present. The anterior articular surface of the centrum is plane, the 

 posterior moderately concave ; both surfaces show a straight superior 

 and equally rounded lateral and inferior border, having thus the shape 

 of a circular disc, of which a segment is missing. 



The articular surfaces for the first cervical rib are situated on the 

 anterior superior part of the centrum, and on the middle of the arch 

 immediately above the neuro-central suture. The neural arch is much 

 mutilated, and only one post-zygapophysis is preserved, which is situated 

 comparatively high above the centrum. Its articular surface is not 

 clearly defined, and is directed outward and downward. 



With the exception of its being a few millimetres shorter, the 

 second vertebra, of which only the centrum is preserved, has 

 practically the same shape as the axis, the chief differences being 

 the stronger development of the hypapophysial knobs and the 

 evidently more elevated pleurapophysis, for no trace of it can be 

 found in the part preserved. Compared with the cervicals of the new 

 0. Lennieri, preserved at the Havre Museum, the description of which 

 will shortly be published elsewhere, there is a great difference in the 

 laterally compressed centra^ of St. prisons and also in the rectangular 

 shape of the neural platform (Hulke), which is sandal-shaped in 

 0. Lennieri, being strongly contracted in the middle. No great 

 difference between the cervicals of our Stegosaurus and the American 

 Stegosaurus is apparent. 



It does not seem improbable that the left- triradiate cervical rib, 

 represented in Fig. lb, belonged to this or to the following vertebra, 

 for the distance of the capitulum and tuberculum would correspond 

 to the proportions we should expect to find in these vertebras if they 

 were complete. 



Besides two processes for the capitulum and tuberculum and the 

 comparatively short and flattened body of the rib, this bone shows 

 on the exterior part a well-marked excrescence, which is represented 

 in most Dinosaurs only by a feeble ridge, but is well developed, though 

 with altered direction, on the cervical rib figured by Marsh as a rib 

 of Apatosaurus. In Stegosaurus this excrescence is pointed outward 

 and forward, and it may perhaps be best compared with the dilatation 

 of the cervical ribs in some Lacertilia. It may have served for the 

 purpose of combining a limited amount of flexibilitj" with strength in 

 this region of the body. A. quite similar, though more ridge-like 

 excrescence is also met with on the thoracic ribs of Stegosaurus, and 

 produces there, together with a similar posterior ridge, the oblique 

 T-shaped cross-section that has been specially noticed in Stegosaurus, 

 but seems, as far as I am aware, to be present also in other members of 

 the Orthopodous order. 1 Its origin may therefore have to be explained 

 otherwise than through the weight of the dermal armour. 



The dorsal vertebrae of St. prisons (Fig. lo) approximate in general 



1 I would suggest that the Dinosaurians represent a distinct super-order, 

 which may be divided into two orders, Saurischia (Seeley) and Orthopoda. 



