Maryland Geological Survey 209 



concavity. The bone tapers decidedly, as the anterior face is less than 

 four-fifths of the transverse diameter of the posterior. The sides are 

 decidedly concave and meet in a slight keel-like angle inferiorly. The 

 neural canal is extremely narrow, especially at a point just in front of 

 the mid-length of the centrum. There are several tiny, irregularly placed 

 foramina which enter the centrum from the bottom of the canal, the two 

 anterior ones being separated by a slight ridge. The irregular ridges 

 on the articular face for the pedicels of the neural arch are approxi- 

 mately parallel, only radiating at the anterior end of the centrum. The 

 bone appears to be solid, of fine cancellous character, with no trace of 

 lateral depressions, such as one finds in Pleuroccelus. This vertebra which 

 evidently came from the mid-dorsal region, compares very closely with a 

 vertebra of Stegosaurus ungulatus (cotype No. 1858, Yale Museum), ex- 

 cept for size. The two vertebrae differ in the much less relative depth of 

 the posterior concavity in Stegosaurus, and in the fact that the anterior 

 and posterior faces are the same diameter in Stegosaurus, but there is a 

 similar, though not quite so marked, tapering of the pedicel facets. The 

 groove of the neural canal is wider in Stegosaurus, and seems to lack the 

 constriction, but it is quite probable that the neural canal widened per- 

 ceptibly vertically before narrowing again in the neural arch of Pricono- 

 don. There is no trace of a keel-like ridge on the lower side of the 

 centrum in Stegosaurus, though the curve of the section is sharper here 

 than on the sides. In neither case is there a trace of the capitular rib 

 facet on the centrum, which in Stegosaurus is high on the greatly elevated 

 neural arch. 



In pi. xvii of the forthcoming monograph of the Stegosauria this bone 

 is figured, together with a longitudinal section, showing its extremely 

 dense cancellous tissue comparable to that of the centrum under discus- 

 sion. From this comparison it seems reasonably sure that Ave have here 

 the centrum of a stegosaurian dinosaur which will be provisionally re- 

 'ferred to Priconodon. Whether it may be referred to P. crassus or not 

 is an open question, for the vertebra is much smaller than that of the 

 adult Stegosaurus ungulatus while on the other hand the teeth of 

 Priconodon are much larger than those of the stegosaur. Leidy says. 



