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STRUCTURE AND RELATIONSHIPS OF OPISTHOCCELIAN DINOSAURS. 177 
: THORACIC RIBS. 
Eighteen ribs, more or less complete, are preserved with this speci- 
men. Ten of these belonging to the right side form a continuous 
series in which the first and last are so reduced in size as to indicate 
that the series is complete. Those of the left side correspond very 
closely with the right, except that the third and fourth have been 
carried away entirely. This number of ribs, preserved so nearly in 
their normal positions, may be regarded as conclusive proof that there 
were but ten vertebre in the dorsal series. 
In the. frst pair of ribs more than half of the distal ends were 
eroded away. The head and the tubercle are slender and divergent; 
the shafts taper toward the distal end and are too slender to have 
performed any important function. (Plate xLvi1.) The second pair of 
ribs is considerably stronger than the first. Only the proximal half of 
the right and the head of the left are preserved. At this point the 
divergence of head and tubercle is most pronounced, the head forming 
an angle of sixty-five degrees with the shaft. The third and fourth 
pairs of ribs are the strongest of the series. The articular facets are 
broad and rugose, and stand at almost aright angle to the shaft. The 
proximal ends are expanded and massive, bearing trough-like fossz 
on their mesial surfaces. The distal ends are missing from both. 
The fifth pair of ribs is complete and well preserved. The head 
is more elongate, indicating greater expansion of the thorax at this 
point. The distal end bears a broad and roughened surface for the 
attachment of the costal cartilage. The right member of this pair is 
of interest in having an enlargement in the shaft, due to an imperfectly 
healed fracture. The adjoining sixth rib has a similar fracture which 
failed to heal, Thesixth pair is marked by a decided reduction in 
size, a noticeable shortening, and a slender and tapering distal end. 
The head continues to elongate, the tubercle to shorten. In theseventh 
and eighth pairs the shafts become shorter and slighter, and the head 
more elongate. In the ninth the head and tubercle are quite reduced 
and the shaft is more curved at the proximal end. The tenth pair is 
reduced to mere rudiments whose shafts are almost lost. The right one 
has co6éssified with the vertebra by the head and tubercle while the dis- 
tal end has a flattened surface which abutted the crest of the ilium. 
MEASUREMENTS OF RIBS. 

PemMNUMBER. 222572 o2 I II III IV Vv Vee ee ay LET. LX xX 
Bier ge feo I es Soe Sec 2,13 |) Sm 1.054, 70 41,20: 02 
etubercle ne ene | 430 [510.500 | .440 | 408 | 420] 360 | .310 | 250] .240 
Breadth of shaft at middle-_ | .o80 .095 | .130| .130 | .125 | .085 | .0g0 | .085 | .080 | .o60 


