CETOTHERES FROM THE MIOCENE CHOPTANK FORMATION 



23 



the distance between the capitulum and the external 

 margin of the tubercuhim is very slightly less than on 

 the second rib. 



Fourth rib. — The length of the ribs of the fourth 

 pair (USNM 23794; pi. 15, fig. 4) has increased (right, 

 698 mm. ; left, 705 mm.) ; the width of the cui-ved shaft 

 has become more uniform from end to end, and the 

 articular area of the tuberculum has been reduced. Al- 

 though the distance (30 mm.) from the capitular facet 

 to the inner (vertebral) margin of the tubercular facet 

 has increased, this end of the rib is actually shorter than 

 on the third rib. Although longer, the shaft of this rib 

 is actually narrower than the third; its compressed 

 sternal end is likewise rugose and pitted. As compared 

 with the external edge the internal edge of the shaft is 

 less noticeably compressed. 



Fifth rib. — No indication of an angle external to 

 the tuberculum is present on the outer surface of the 

 shaft of either rib (USNM 23794; pi. 15, fig. 5). The 

 end-to-end curvature of the shaft is about the same 

 as that of the preceding rib. At the vertebral end of the 

 shaft, the tuterculum is a saddle-like depression sep- 

 arated from the more nearly cii'cular capitular facet 

 by an inter\^al of 15 mm. Shortening of the neck and a 

 narrowing of the shaft have accompanied elongation 

 of this rib (right, 728 mm.; left, 725 mm.). The attenu- 

 ated sternal end of the right rib is deeply pitted, pre- 

 sumably for attachment of a ligament; this end of the 

 shaft of the left- rib is attenuated but otherwi.se normal. 



Sixth rib. — A slight indication of an angle 120 mm. 

 external to the tul>ercidmn is present on the outer sur- 

 face of the .shaft of the left rib; (USNM 23794; pi. 15, 

 fig. 6). Each of these ribs is slightly shorter (right, 705 

 mm.; left, 715 mm.) than the preceding, although the 

 end-to-end curvature is about the same. The attenuated 

 sternal end of the left rib is deeply pitted, but the sternal 

 end of the left is normal. The saddle-like tubercular 

 facet is separated by an interval of not more than 15 

 mm. from the convex capitular facet. Tlie length of the 

 neck is approximately equal to that of the preceding rib. 

 The outer surface of this rib is slightly convex and the 

 internal is flattened. 



Seventh rib. — The ribs of the seventh pair (USNM 

 23794; pi. 15, fig. 7) are slightly shorter than the sixth, 

 the right and left ribs measuring, respectively, 697 and 

 706 mm. ; the curvature of the shaft is quite similar. A 

 definite angle is developed 140 mm. externa] to the 

 tuberculum on the left but not on the right rib. On this 

 left rib the oval articular face of the knob-like capit- 

 ulum is located at the extremity of the more noticeably 

 constricted and shortened neck; the capitulum of the 

 right rib is much smaller and lacks projecting edges. 



The tuberculum on both ribs is an ill-defined irregularly 

 depressed surface. A slight enlargement attributable to 

 osteophytosis has deformed the left rib about 315 mm. 

 distant from the articular face of the capitulum. The 

 sternal ends of both ribs are compressed and gradually 

 attenuated. 



Eighth rib. — A definite angle is present 240 mm. ex- 

 ternal to the articular face of the capitulum on the outer 

 surface of the left but not on the right rib (USNM 

 23794; pi. 15, fig. 8). The vertebral end of each rib 

 in this pair is quite unlike any of the preceding ribs; 

 the neck is dorsoventrally compressed and attenuated 

 to the small terminal capitulum. The ill-defined tuber- 

 culum is located on an elevation at the commencement 

 of the neck. The shaft of the eighth rib is also more slen- 

 der and much less compressed than that of the seventh 

 rib, but is attenuated at the sternal end. 



Ninth rib. — The right and left ribs of the ninth pair 

 (USNM 23794; pi. 15, fig. 9) measure, respectively, 

 650 and 662 mm. in a straight line; the shaft of each 

 rib is less curved, particularly near the vertebral end, 

 than the eighth rib. The articular area on the vertebral 

 end of the rib is exceedingly nodular and irregularly 

 excavated. 



Tenth rib. — Both ribs of the tenth pair (USNM 

 23794; pi. 15, fig. 10) have their distal ends broken off 

 and lost. Both ribs have the vertebral end dorsoven- 

 trally compressed and widened for the elongated articu- 

 lar surface. The shafts of these ribs are slightly curved 

 and are less compressed than the preceding ribs. 



Eleventh rib. — The narrow articular head of each 

 of the eleventh ribs (USNM 23794; pi. 15, fig. 11) is 

 ovoidal in outline, pitted, and roughened for the liga- 

 ment that serves for attachment to the extremity of the 

 transverse process of the eleventh dorsal. The left rib, 

 which measures 588 mm. in a straight line, may either 

 have been fractured 190 mm. above the compressed 

 distal end or malformed by osteophytosis. 



STERNUM 



Recent Mysticeti possess a sternum consisting of a 

 broad flattened presternum, which is extended posteri- 

 orly into a xiphioid process in some species; but no 

 mesosternal segments have been observed and, conse- 

 quently, only the first pair of ribs are attached to the 

 presternum. The sternum of these Recent mysticetes 

 varies from heart-shaped, longitudinally oval, to tri- 

 lobate. Although it is generally accepted that the ster- 

 num does not provide a satisfactory basis for discrim- 

 inating closely allied species of Recent Mysticeti, 

 evidence now exists that two Miocene genera have a 

 sternum of almost identical shape. 



