AND AVES OF NORTH AMERICA. 



209 



The species are as follows : 



« Articular extremities of dorsal vertebrae depressed. 

 Cervicals much depressed and with smaller articular faces than dorsals ; inferior 

 aspect of cervical centra keeled medially and laterally. B. PLATYSPONDYLUS. 



Dorsals with transversely oval cups, less depressed and less elongate. 



B. FRATERNUS. 



BAPTOSAURUS PLATYSPONDYLUS, Marsh. 



MaavoaauTVB platytpondyltu, Marsh, Proceed. Amor. Assoc. Adv. Science (Salem), 1869. Haluaurus platyspon- 

 dylus, Marsh, SiUiman's Journ. Sci. Arts, 180!), p. 895. 



This most distinct speoies was established <>u a median cervical, and an anterior dorsal vertebra; the right splenial 

 bone with its concave articular face, and a small portion of the base of the skull. A ooracoid found near these speci- 

 mens probably belonged to the same animal, although it is smaller than the other parts of the skeleton would appa- 

 rently indicate, and differs considerably in form and proportions from the corresponding bone in both Liodon 

 and OUdastes. The splenial bone differs from that of Mosasaurus in the form of its articular face, arid in being more 

 overlapped in front of the joint of the dentary. 



Through the kindness of Prof. Marsh, I have had the opportunity of making the present observations. Other 

 parts of the description are copied from his description quoted above. 



The cervicals present smaller cup and ball in proportion to those of the lumbars, than is seen in any other known 

 Mosasauroid; they arc so" depressed as to be nearly twice as broad as deep. The inferior, horizontal limb of the diapo- 

 physis is prominent below, leaving on each side of an obtuse median keel, a lateral shallow concavity. The median 

 keel scaroely projects beyond the lateral. The neural arch is slightly more contracted above antero-posteriorly, than 

 in any other species, hence the zygapophyses are more prominent. The neural spine is contracted antero-posteriorly 

 and much elevated. The hypapophysis is on the posterior portion of the inferior surface, and is quite prominent, but 

 shorter than the centrum. The posterior dorsals resemble only those of Liodon validus out of the genus; they are 

 considerably more slender, and without the obtusely keeled form of the inferior face, characteristic of the latter. 



The dimensions of the posterior cervical vertebra, are as follows: 



Inches. Lines. 



Length of centrum below, 3 fi 



Transverse diameter of articular cup, 1 8.5 



Vertical diameter of articular cup, 11. 



Distance from centre of cup to middle of lateral edge, 11. 



Length of diapophyses below lower margin of oup, 5.5 



" " hypapophysis below lower margin of ballj 8. 



The dorsal vertebra has no trace of hypapophysis, and the lower surface of the centrum is remarkably flat. The 

 anterior downward prolongations of the diapophyses still project, below the centrum, but they have become obtuse pro- 

 cesses, partially separated from the more vertical portion of the diapophyses by a notch. The following arc the more 

 important, dimensions of this vertebra,: 



Inchet. 

 Length of centrum below, 3 



Transverse diameter of cup, 1 



Vertical " " 1 



Width of neural (-anal below, 



" between extremities of anterior zygapophyses, 2 



Length of diapophyses below lower surface of centrum, 



AMK1UOA. PHILO. SOC. — VOL XIV. 53 



Line). 

 6. 

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