E. S. Riggs — Brachiosaurus altithorax. 303 



are deeply hollowed by lateral cavities. The whole character 

 of their sculpturing tends toward a lightness bordering upon 

 fragility. In this particular they are approached most nearly 

 by the vertebrae of Diplodocus. 



The sacrum is composed of four firmly coalesced vertebrae. 

 Its most distinctive feature is its great breadth in comparison 

 with its length. The measurement across the anterior end of 

 the transverse processes is one-fourth greater than that of the 

 posterior end and one-third greater than the type specimen of 

 Brontosaurus. The first sacral rib arises from the anterior 

 end of the centrum and is relatively unimportant. The second 

 arises from the whole length of the centrum and is by far the 

 strongest of the series. Its iliac articular surface is much 

 expanded and marks the axial point in the sacrum. The third 

 and fourth sacral ribs arise from the anterior half of their 

 respective centra. As neither the sacral nor presacral verte- 

 brae have yet been removed from the matrix, the description 

 of their spinal elements will be deferred until a future publica- 

 tion. 



The anterior caudal vertebrae are amphicoelian in form and 

 relatively small in comparison with those of other Sauropoda. 

 The anterior faces of the centra are more deeply concave than 

 the posterior faces. Since the vertebrae were found lying upon 

 their sides in close apposition with the sacrum, the marked 

 posterior concavity can hardly be attributed to distortion. 

 Unfortunately the spine in caudal I is not preserved. In 

 caudal II the neural arch is as simple as in caudal VIII* of 

 Brontosaurus and the spine is scarcely as long. There is no 

 trace of lateral cavities in the centra, or of the broad vertical 

 plates developed from the diapophyses in that genus. On the 

 contrary, these lateral processes are simple, peg-like prominences 

 slightly flattened vertically. The zygapophyses are imperfectly 

 preserved in this specimen, but were apparently slight. The 

 neural spine is short, stout, laterally compressed at the middle 

 but expanded into a rugose knob at its extremity. 



The unusual length of the ribs bears evidence of the immense 

 thorax of this animal. In the mid-thoracic region they measure 

 fully nine feet (2'745 m ) in length. The capitulum and tuber- 

 culum are almost equally developed and widely separated, to 

 give the firm anchorage necessary to the great length of the ribs 

 (fig. 5). In some instances the attachment is strengthened by a 

 second tubercle on the posterior surface of the head similar to 

 that figured by Marshf in the cervical ribs of Apatosaurus. 

 The anterior surface of the shaft below the head is perforated 



* Caudal VII according to Marsh's restoration, 

 f The Dinosaurs of North America, p. 167. 



