Maryland Geological Survey 193 



Vertebral Column. — The cervical and dorsal vertebrge are elongate 

 and strongly opisthocoelous, though the dorsals evidently became shorter 

 as they approached tlie sacrum and became less opisthocoelous as well. 



The vertebrae are all characterized by deep lateral cavities, so deep 

 that they are separated only by a thin septum of bone in the mid-line. 



Throughout the entire presacral series the vertebrae are relatively 

 much longer than in Morosaurus, the comparison being made with a 

 specimen of the latter of equivalent individual age, Morosaurus latus, 

 type (No. 1910, Yale Museum), and the latter cavities are also longer, 

 though bearing about the same relation to the length of the centrum. 



Many vertebras have been discovered, but relatively few of them can 

 be placed with certainty, as no two can be proven to have belonged to 

 the same animal. A few, however, may be taken as examples. 

 / The anterior cervical (No. 5700, IT. S. National Museum), which 

 appears to be the axis, is the slenderest of all the known vertebrae of 

 Pleuroccelus, differing from that of Morosaurus in being more slender 

 with the lateral cavities longer, but less deep. Anteriorly, the bone is 

 corroded away, giving it decidedly the appearance of an odontoid process, 

 though the latter shows no sign of sutural union with the body of the 

 vertebra, as in the young Morosaurus lentus, which also shows the 

 intercentrum of the axis as a separate element. The neural canal is 

 broad. The posterior cavity is less deep than in the posterior cervicals. 



DIMENSIONS 



Length 68.5 mm. 



Breadth anterior end 32.0 mm. 



Breadth posterior end 37.0 mm. 



Height of centrum 33.0 mm. 



In the posterior cervical (No. 5678, U. S. National Museum), which 

 is that described by Marsh, the pleuroccelous condition seems to have 

 reached its maximum. The lateral depressions are very deep and extend 

 fore and aft into pocket-like cavities, so that the entire centrum is 

 reduced to a shell. The neural canal is broad, but widening out, espe- 

 cially in the rear, and with a slight median longitudinal ridge on its 

 floor. The sutural surfaces for the attachment of the neural arch are 



