62 
Thoracic Ribs. Of the 
Figure 33. Right rib of ?fourth or 
fifth dorsal vertebra of paratype of 
Edmontosaurus; anterior view obli- 
quely from without; i natural size. 
h, head; t, tubercle. 
pair of ribs assigned to the anterior dorsal 
vertebra (?fourth or fifth) the one of the 
right side is complete and well preserved, 
the left one is in an equally good state of 
preservation but lacks a short piece from the 
lower end. 
These anterior ribs (Figure 33) are long 
and strongly built with a moderately curved, 
tapering body or shaft, a well-developed 
head and tubercle, and a deep, thin neck. 
In the upper half of the shaft the bone is 
thick with a rounded border along the antero- 
internal curve and down to the head; along 
the postero-external curve and between the 
tubercle and the head it comes to a rather 
thin edge. The broadest part of the rib is 
below the tubercle. The posterior face in the 
upper half of the shaft, and for a short dis- 
tance past the tubercle toward the inner 
border of the neck, is transversely concave. 
For a corresponding distance on the ante- 
rior face the surface is transversely convex, 
the convexity developing down the shaft into 
a well-defined median ridge which merges 
farther down into the antero-internal border. 
It is along this ridge that the bone is thickest. 
In the lower half of the shaft the bone 
becomes more nearly ovate in cross-section 
being somewhat thicker near the outer curve 
with the greatest diameter directed externo- 
internally. Along this lower portion of the 
shaft the rate of taper is lessened. At the 
extreme lower end the bone thickens and 
there is a roughened surface for the attach- 
ment of the costal cartilage. The neck is 
directed downward, inward, and slightly for- 
ward at an angle of about 1 10 degrees to the 
upper part of the shaft. 
The head of the rib is set rather squarely 
across the neck and expands to a thickness 
which is twice that of the neck near its lower 
border. Its articular surface is undulating, 
pitted, and rugose with an irregular lengthened 
oval outline about twice as deep as broad. 
The tubercle is prominent but much 
smaller than the head. Its articular surface 
is about half that of the head in area and is 
set at much the same angle. From the point 
of view at which the rib has been drawn 
neither of these articulating surfaces is fullj^ 
seen in Figure 33. 
