. Bulletin. Natural history. 160 PEABODY MUSEUM BULLETIN 23 Distribution. DeGrey Member, Lower Pierre Shale, South Dakota. Diagnosis. Large prominence projects posteriorly beneath stapedial pit on quadrate, nearly closes stapedial notch from below. Anterior zygopophysis of cervical and anterior dorsal vertebrae connected to synapophysis by gently rounded, posteriorly descending. Text-fig. 87. Medial view of right quadrate of type of Plioplatecarpus primaevus (USNM 18254, X iA). crest. Functional zygopophyses probably end near posterior end of dorsal series, or in pygal region. Posterior border
. Bulletin. Natural history. 160 PEABODY MUSEUM BULLETIN 23 Distribution. DeGrey Member, Lower Pierre Shale, South Dakota. Diagnosis. Large prominence projects posteriorly beneath stapedial pit on quadrate, nearly closes stapedial notch from below. Anterior zygopophysis of cervical and anterior dorsal vertebrae connected to synapophysis by gently rounded, posteriorly descending. Text-fig. 87. Medial view of right quadrate of type of Plioplatecarpus primaevus (USNM 18254, X iA). crest. Functional zygopophyses probably end near posterior end of dorsal series, or in pygal region. Posterior border of scapula strongly emarginated posteromedially. Coracoid foramen small. Discussion. Plioplatecarpus primaevus is confidently referred to Plioplatecarpus on the basis of the characteristic shape of its quadrate, the elongate, gently-convex superior border of the scapula, and the greater size of the scapula when compared to the cora- coid. It differs widely from P. marshi in the larger size of the posterior prominence beneath the stapedial pit on the quadrate, its much smaller coracoid foramen and in the presence of a posteromedial emargination on the posterior border of the scapula (see Dollo, 1904a, pi. 6; 1882, pi. 6). The quadrate of P. primaevus is very similar to that of P. houzeaiii, but the former species differs in that the zygopophyses are well developed in the anterior dorsal region while in the latter they disappear on die third dorsal (Dollo, 1894, p. 235). In P. primaevus the anterior zygopophyses of the anterior dorsal vertebrae are linked posteriorly to the synapophyses by a strong crest that is completely lacking in P. depressus. It is seen that, except for the absence of zygosphene-zygantra, the anterior vertebrae of P. primaevus are nearly identical to those of Platecarpus. It is likely tlierefore, that tliis species had a presacral count closer to 29 vertebrae, as in Platecarpus, than to the reduced number of 20 found in P. houzeaui. Plioplatecarpus primaevu
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