OSTEOLOGY OF CARNIVOROUS DINOSAURS. 53 



extend nearly straight downward, the median and posterior ribs form a more barrel- 

 like thoracic cavity. These progressive changes from first to last, may be observed 

 in the ribs illustrated in figure 36. 



The external surface of the second rib is narrow and bladelike proximally, but 

 on the third this blade is produced posteriorly, thus presenting a broader aspect 

 to the outer face, which becomes more pronounced in the succeeding ribs. In 

 cross section it would be the form of an inverted L. With an increasing slender- 

 ness downward they become sub triangular, then subcircular, and finally narrowly 

 oval. Some of the ends in the mid-thoracic region as shown by ribs of the opposite 

 side are slightly enlarged. 



The longest ribs of the series appear to be in the mid-dorsal region, probably 

 the sixth, posterior of which they commence to shorten. The most posterior rib is 

 beneath the anterior crest of the ilium as in Ceratosaurus. The length of the ribs can 

 not be given because of the missing distal ends. 



Abdominal ribs. — The presence of abdominal ribs in Antrodemus was first made 

 known by Osborn 1 in 1906. In plate 15 is shown the abdominal ribs belonging to 

 to specimen No. 4734, U.S.N.M., as they were found in the rock. Unfortunately 

 these are so disarranged and 

 their state of preservation so 

 incomplete as to make it im- 

 possible to determine the total 

 number of these ribs, or the 

 shape of the cuirass which they 

 formed . Lamb e 2 has recently 

 siven a most interesting and ^ IG- 37- — double abdominal rib of antrodemus valens leidt, no. 4734, 



. . 1-11, U.S.N.M. J NAT. SIZE. 



instructive detailed account 



of the abdominal ribs in Gorgosaurus libratus. A comparison of the abdominal 

 ribs before me with Lambe's excellent illustrations and descriptions appears to 

 indicate only a general resemblance in the structure of these bones. They 

 occur in a longitudinal series, of which parts of 14 from one side, probably 

 the right, are in sequence in the rock as shown in plate 15. In Gorgosaurus, 

 Lambe provisionally regards the full series of one side to consist of 19 ribs. In 

 that genus each rib consists of two parts, an elongated bone with an enlarged and 

 flattened inner end for overlap with the rib at the opposite side at the center, and a 

 shorter, slender, tapering, rodlike bone, the greater portion of which is applied along 

 the side of the outer end of the large bone. Portions of similar paired bones are 

 clearly shown in the photograph of these elements as found in the sandstone, see 

 plate 15. Only one (fig. 37) of the double abdominal ribs, such as are present in 

 Gorgosaurus and Tryannosaurus , was found with this specimen. 



Associated in the rock with the bones described above are a number of short 

 rodlike pieces of bone, varying in length from 80 to 110 mm. with slightly expanded 

 ends that I will presently attempt to show are additional segments of the abdominal 

 ribs. 



1 Osborn, Henry Fairfield, Bull. Amer. Mus. Nat. Hist., vol. 22, p. 283, 1906. 



2 Lambe, L. M. Memoir 100, Canadian Geol. Surv., No. 83, Geol. ser., 1917, pp. 37-44. 



3 Osborn, H. F., Bull. Amer. Mus. Nat. Hist., vol. 22, p. 295, fig. 12. 



144035°— 20 5 



