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University of California. 



[Vol. 3 



dimensions are nearly equal and the ci'oss-section circular or pear- 

 shaped. In the caudal region the centra are laterally compressed 

 and elliptical or pentagonal in cross- section. 



The upper arches are slightly longer in the dorsal than in the 

 cervical or caudal regions. Throughout the whole column they 

 show a more or less pronounced tendency to the development of 

 a heavy, lateral rib, produced by a sudden thickening of the 

 median portion of the spine. 



In the fifth and sixth cervicals of alexandrce the spines are 

 very long and practically circular in cross-section near the sum- 

 mits. A loose cervical arch of osmonti shows a similar section. 

 The anterior spines of the perrini specimen are somewhat thinner 

 than those of osmonti but are probably not from the most anterior 

 portion of the column . In what is taken to be an anterior dorsal 

 arch of osmonti (PL 9, figs. 3 to 5) the antero-posterior diameter 

 just above the posterior zygapophyses is 25 mm. and the thick- 

 ness 13 mm. In an individual from the middle dorsal region the 

 arch is 35 wide and 15 thick. In perrini the spines of the middle 

 dorsals are much wider than in the anterior portion of the column, 

 but continue to show the median thickening. In the type species, 

 pacificus, (PI. 1-4, fig. 1) distinct ribbing appears on what are 

 probably posterior dorsals. In altispinus (PI. 15, figs. 3 and 4) 

 the median or posterior dorsals seem to have higher and relatively 

 thicker and narrower spines than in any other form. Excepting 

 the blade- like anterior and posterior margins, their cross-sections 

 are almost circular. Particular notice has been taken of this 

 ribbing by Dames (1895, p. 1015) who calls attention to its 

 presence in Mixosaurus . Either ribbing or a decided thickening 

 of the arch would certainly seem to be a characteristic of Triassic 

 Ichthyopterygia. 



In the anterior dorsals and cervicals the pedicels of the upper 

 arches are greatly extended laterally, so that they almost, if not 

 quite, touch the ribs. This gives them an elongated triangular 

 form at the base. The surfaces on the centrum, above which the 

 upper arches rest, are excavated and considerably roughened. 

 The excavation is extended laterally as a shallow groove upon the 

 upper portion of the diapophysial ridge in the cervicals and 

 anterior dorsals (PI. 9, fig. 1). In the posterior dorsal region 



