OSTEOLOGY OF POLYGLYPHANODON—GILMORE 247 
Dorsal vertebrae.—Including the one that is missing, the presacral 
series in specimen U.S.N.M. No. 15816 (pl. 25) consists of 23 vertebrae. 
The detailed description to follow is based, however, to a great extent 
on the vertebrae of specimen U.S.N.M. No. 15566, since they have been 
more completely freed from the matrix than those of the paratype. 
Seen from below, the centra are tapering in outline, depressed, with 
subovate condyle and a glenoid fossa. Nowhere is there evidence of 
a ventral keel or carina, the ventral surface being broadly rounded from 
side to side in the anterior part of the dorsal series and flattened in the 
posterior part. The series is greatly similar in structure throughout. 
The first and second dorsals have narrowed spines, but beginning on 
the third the neural spines are wide anteroposteriorly, with a nearly 
vertical posterior border and a long sloping anterior border. This 
style of spine, of uniform height, prevails to the twentieth vertebra. 
On the three posterior dorsals the anteroposterior diameter of the top 
portion of the spines suddenly contracts, and the tops are squarely 
truncate, with a very slight transverse expansion of this end. 
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Ficure 24.—Median dorsal vertebra of Polyglyphanodon sternbergi Gilmore, U.S.N.M. 
No. 15566, natural size: A, Lateral view; B, ventral view; C, anterior view; D, dorsal 
view. d, Diapophysis; z, anterior zygapophysis; 2’, posterior zygapophysis; zyg, zygan- 
trum; zys, zygosphene. 
The centra to the middle of the thoracic series gradually lengthen 
and then posteriorly continue with little change in length as far as 
the twenty-second dorsal. The twenty-third has the centrum decidedly 
shortened, as shown by the type specimen, which has the twenty- 
second, twenty-third, and sacrum articulated (see fig. 26). For exam- 
ple, measured along the center of the ventral side from the edge of the 
cup to the beginning of the ball the twenty-second is 14 mm., whereas 
the twenty-third or last dorsal is only 11 mm. in length. 
Viewed from the side a prominent rounded ridge extends from the 
base of the diapophyses diagonally downward and backward, merging 
