22 



UNITED STATES NATIONAL MUSEUM BULLETIN 294 



RIBS 



Eleven pairs of ribs were associated with this skeleton 

 (USNM 23794). No trace of a twelfth pair of ribs was 

 found at the site of excavation. The lack of a roughened 

 area on the beveled obliciucly trwiicuted extremity of 

 the transverse process of the twelfth doi-sal vertebra 

 seemingly suggests but does not necessarily confirm the 

 absence of a twelfth pair of ribs. 



The capitulum at the vertebral end of the first to 

 eighth pair of ribs, inclusive, is lodged mainly in the 

 protuberant posterior facet on the dorsocxterniil fac^ of 

 the centrum of the preceding vertebra. The tuberculum 

 of each of the eight anterior pairs of ribs articulates 

 with the facet at tlie extremity of the diapophysis. The 

 rather robust neck of the ribs in the eight anterior pairs 

 progressively diminishes in length from the second (the 

 longest) to tlie eighth. On tlie first rib, the vertebral end 

 is strongly compressed anterposteriorly and the distance 

 between the vertebral margin of the tuberculum and the 

 external margin of the capitulum is short (right, 20 

 mm.). The single head of the ninth, tenth, and eleventh 

 ribs is asymmetrical, and articulates solely with the 

 facet at the extremity of the transverse proce-ss (para- 

 pophysis). 



As a result of curvature the anterior face of the 

 shaft near the vertebral end imperceptibly bei-omes the 

 external face toward the distal end on all the ribs be- 

 hind the fii-st pair; this face is more convex than the 

 posterointernal face. 



The first rib is the shortest luid most strongly com- 

 pressed anteroposteriorly, and also most abniptly curved 

 inward at the ventral or sternal end of all the ribs. 

 This rib most certainly had a ligamentary attachment 

 to the sternum. Behind the first pair the ribs i)rogres- 

 sively increase in length (m a straight line) to the fifth 

 and sixth pairs ajid then decrease to the eleventh pair. 

 The distal or sternal ends of the second, third, and 

 fourth ril)s on tlie right side, and the second, third, 

 fourth, fifth, and sixth on the left side are tnmcated 



and rugose, possibly for attaclinaent of some ligamen- 

 tary connection with stenial ribs. The manubrium is 

 much too small to provide a base for attachment of so 

 many ribs. All other ribs are attenuated toward the 

 ventral or distal end and presimiably terminated freely 

 in the mass of abdominal muscles. See table 16 for 

 measurements of the ribs. 



First rib. — The distal end of the shaft is broken 

 off on both the right and left, ribs (USNM 23794; pi. 

 15, fig. 1). On the right rib the small ovoidal capitular 

 facet is separated from the much larger tubercular 

 facet by a short interval (20 mm.). Both of these facets 

 are malformed on the left rib. The shaft of these ribs 

 is markedly compressed anteroposteriorly from the ver- 

 tebral to the sternal end, and also strongly curved. 

 Since the seventh cervical and the diapophysis of the 

 first dorsal were not recovered with this skeleton, the 

 articular relations of this first rib are uncertain. 



Second rib. — On the right side, this long rib (USNM 

 23794; pi. 15, fig. 2) is complete; the vertebral end of 

 the left rib is broken off. This rib is considerably larger 

 than the first rib ; its rugose sternal end is compressed 

 but not reduced in width and presumably was attached 

 to the sternum by a ligament. The flattened capitular 

 facet is located at the end of the anteroposteriorly com- 

 pressed neck and is separated by an inten'al of 55 mm. 

 from the concave articular surface of the tuberculum. 

 No angle is developed on the shaft external to the 

 tuberculum; the curvature is regular, the external edge 

 being thicker than the internal. 



Third rib.— The third pair of ribs (USNM 23794; 

 pi. 15, fig. 3) are more curved and longer than the 

 second, the right and left measuring, respectively, 637 

 and 636 mm. ; the shaft is strongly compressed toward 

 the sternal end. The sternal end of the shaft is rugose 

 and pitted, and not. appreciably narrower than the 

 widest portion of the shaft. At the vertebral end of 

 the neck, the distance between the large ovoidal capitu- 

 lar facet and the narrow elongated tubercular facet 

 (left, length, 60 mm.) is reduced to 18 mm., although 



Table 16. — Measurements (in mm.) of ribs, USNM S3794 



31 



35 



34 



