AET. 16 FOSSIL EEPTILES PROM MONTANA GILMOEE 21 



being much the stouter. The shaft is flattened, the distal fourth 

 curved slightly inward and terminated with a bluntly rounded end. 

 The external surface is angularly convex transversely. On the 

 external surface there is a longitudinal ridge-like roughening of the 

 bone which occupies its distal two-thirds, as shown in Figure 14B. 

 The proximal half of a larger rib, probably the fourth or fifth of 

 the left side, records the following changes: The capitular process 

 has become much longer than the tubercular, the shaft is more 

 rounded, and the posterior side deeply concave. On the external 

 surface the roughened longitudinal area has assumed a somewhat 

 lower position on the shaft ; the distal end is missing. The proximal 

 has a spread of 101 millimeters as compared with 70 millimeters in 

 the second. 



The proximal end of a right rib farther back in the series displays 

 a decided lengthening of the capitular process which is slender and 

 nearly circular in cross section. A right rib from the posterior part 

 of the cervical series much curved from end to end is illustrated in 

 Figure 15C. The distal extremity is missing, but from the tapering 

 character of the bone above the break it is quite evident that it was 

 pointed. On the upper half the posterior margin extends backward 

 from the heavier shaft, forming a deep longitudinal concavity on the 

 posterior side. This rib, measured over the curve from the top of 

 the tuberculum, is estimated to have a length of 375 millimeters and a 

 width at the center of 40 millimeters ; the broken distal end measures 

 11 millimeters antero-posteriorly. 



A complete anterior rib of the left side which may possibly repre- 

 sent the first throracic rib is illustrated in Figure 15B. That it may 

 have been joined to the sternum is indicated by its slightly enlarged 

 distal end. In cross section below the tuberculum, the shaft has a 

 peculiar shape in so much as the usual elliptical rib has its outer 

 surface developed posteriorly into a thin flange that forms a deep 

 longitudinal concavity on the posterior side. The tuberculum is 

 merged into and extends inward on the upper edge of the capitular 

 process, which is nearly as wide and heavy as the shaft itself. This 

 bone, measured over the curve from the tuberculum, has a length of 

 510 millimeters. 



The longest rib of the entire series is shown in Figure 15A. It 

 measures 890 millimeters in length and is heavy and massive through- 

 out. The upper portion is T-shaped in cross section. At a point 

 midway between the middle and the distal end is an enlarged rough- 

 ened area which morphologically appears to correspond to the posi- 

 tion of an uncinate process. Whether these processes existed as dis- 

 tinct elements in the armored Dinosauria has not yet been determined. 

 Nopsca ^° attributed the flangelike overhang of the anterior thoracic 



" Geologica Hungarica, Budapest, vol. 1, 1928, p. 56. 



