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STRUCTURE AND RELATIONSHIPS OF OPISTHOCGLIAN DINOSAURS. 239 
point a rounded ridge passes obliquely downward and backward to 
the base of the greater peduncle. The points of attachment of the 
second and third diapophyses are very slight; no trace of a fourth 
appears. The fifth has a strong attachment at the thickened crest 
above the lesser peduncle. The plates connecting the diapophyses 
with the sacral ribs do not attach to the iliac wall as in A patosaurus. 
THORACIC.RIBS. 
The unusual length of the’ ribs, as well as the breadth of the head 
and tubercle and the strength of the shaft, bears evidence of the 
immense thorax of this animal. One of the more slender ribs from 
the mid-thoracic region measures fully nine feet (2.745 m.) in length. 
Another has a shaft eight inches (.204m.) in breadth. The head 
and tubercle are almost equally developed and widely separated to 
give the firm attachments rendered necessary by the great length of 
the ribs. (Fig. 5, Plate LX XV.) In some instances the attachment 
is strengthened by a second tubercle on the inferior surface of the 
head similar to that figured by Marsh on the cervical vertebre of 
Apatosaurus. The anterior surface of the shaft below the head is 
perforated by a large foramen which leads to an internal cavity. 
On account of the elevation of the capitular facet. on the vertebra, 
the head and tubercle are borne almost on a level. By reason of 
this the flattened surface of the proximal end passes insensibly into 
the lateral surface of the shaft without that twist common to the 
ribs in animals of this group. 
LEG .BONES. 
The humerus is somewhat crushed antero-posteriorly and twisted 
so that the head and distal end are brought into the same plane. 
The surface of the distal end has flaked away in the process of weath- 
ering to a firm chalcedony core. (Fig. 1.) In relative slenderness the 
humerus is approached more nearly by that of Diplodocus than that 
of any other well-known American dinosaur. The head is consider- 
ably expanded, forming a rounded prominence especially conspicuous 
on the posterior surface. The great tuberosity is stout and rugose; 
its proximal surface meets the lateral margin of the shaft in a pro- 
nounced angle. This angle is not produced posteriorly to inclose a 
fossa as in Morosaurus. The inner border below the head is drawn 
out into a rather thin margin, though roughened for muscular attach- 
ment. The deltoid crest is partially broken away, but was evidently 
quite prominent. (Fig. 4, Plate LX XIV.) Its base forms with the 
anterior surface of the shaft a broad and shallow concavity. Mid- 
