8 



UNITED STATES NATIONAL MUSEUM BULLETIN 294 



have been about one-fiftli (19.5 percent) of tlie entire 

 lengtli of tlie skeleton of this Choptaniv niysticete. 



The pectoral flipper of this Miocene Choptank 

 cetotliero was undoubtedly capable of being elevated, 

 depressed or tilted. PLxcept for the ball and socket articu- 

 lation of the scapula and the head of the humerus, all 

 other joints of the foreliinb had lost their .synovial char- 

 acter and were assuredly joined together and stiffened 

 by fibrous interosseous tissue. This fibrous tissue did, 

 however, give considerable elasticity to the nonjointed 

 flipper. The elbow had become incapable of bending. 

 Bending of the elbow seems not to have been needed 

 during either i)ropulsion or balancing. 



The scapula of these Miocene mysticetes conforms in 

 general structure to that of the Eocene Basilosaunis. 

 This fan-shaped .shoulder blade has a prominent acro- 

 mion, usually a somewhat smaller coracoid process, and 

 the crista scapulae (spine) is limited to a vertical ridge. 

 Since the forelimb of the mysticete is employed as a 

 steering oar, the necessary surface for attachment of the 

 muscles that provide the leverage for manipulation of 

 this flipper are differently positioned than on the hu- 

 merus, radius, and ulna of a land mammal. The radius 

 and ulna are compressed and lack obvious muscle crests 

 as well as grooves for ligaments. In position, the radius 

 is situated in front of the ulna, an arrangement that 

 existed during the Eocene in both Biiuiilomurm and 

 Zygorhha. The distal trochlea on the archaeocete hu- 

 merus, however, has been replaced by anterior and pos- 

 terior contact surfaces on the exterointernally com- 

 pressed distal end of the mysticete humerus, providing 

 a stifl'ened connection with the radius and ulna. The 

 metacarpals of Recent mysticetes, which are compressed 

 in an extensor-flexor direction almost to the same extent 

 as the phalanges, are readily distinguishable by being 

 thicker, more cylindrical and less flattened in the nianus 

 of this Choptank whale. The number of phalanges have 

 undoubtedly increased by reduplications above the usual 

 three in the fingers of land mammals, resulting in an 

 elongation of the three middle digits. 



Five of the six carpal bones associated with the right 

 forelimb were found lying in a nearly normal position 

 at the extremities of the radius and ulna. Tliese carjial 

 bones are ossified and have an irregularly roughened 

 circumference which is indicative of attachment of 

 fibrous tissue. The flattened area of the inner (flexor) 

 surface of each carpal is larger than the outer (extensor) 

 face. A narrow flipper, similar to that of the living 

 balaenopterids is suggested by the spacing of the radiale, 

 intermedium, and ulnare. Among Recent mysticetes os- 



sification of these carpal bones has been retarded to such 

 an extent that they are represented by relatively insig- 

 nificant centers of ossification in the hyaline substance, 

 althougli in old individuals these bony centers are visible 

 on the surface of the cartilage. 



One of the four metacarpals of the right manus has a 

 somewhat shorter shaft than the others, but unfortu- 

 nately the proximal epiphyses of all except one were de- 

 tached and missing. Flattening of the shaft has com- 

 menced but does not materially alter its shape. These 

 metacarpals are distinguishable from the more notice- 

 ably flattened phalanges. The number of digits is 

 uncertain. 



SCAPULA 



The large fan-shaped scapula (USNM 2379-1; pi. 3, 

 fig. 1 ) has a broad acromion, a short, stout coracoid proc- 

 ess and a very narrow prescapular fossa which is 

 progressively reduced below the anterovertebral angle 

 toward the acromion ; the ratio of heiglit to the greatest 

 width is about 3 to 5. The spine is represented by a ridge 

 that extends upward from the dorsal basal edge of the 

 acromion and fades into the external surface about 40 

 mm. below the vertebral margin. No other external 

 ridges are visible. The broad blade, which is thickened 

 along the anterior border internal to the acromion and 

 aLso along the jjosterior border becomes quite thin 

 toward the curved vertebral edge. The posterior edge of 

 the scapula above the articular end is nearly straight. 

 The glenoid articular cavity is deeply concave, broader 

 posteriorly than anteriorly. 



The broad acromion is strongly compressed from side 

 to side, rounded at the extremity and directed forward 

 but not upward. The stout coracoid process also projects 

 forward, but obliquely inward. 



See table 5 for measurements of the scapula. 



Table 5. — Measurements {in mm.) of the scapula, USNM 23794 



Right Left 



Greatest anteroposterior diameter of scapula 492 488 ± 

 Greatest vertical diameter, articular head to 



vertebral margin 310 300 ± 



Length of acromion 90 - 



Posterior angle of blade to end of acromion 470 - 

 Length of coracoid process, dorsal margin at base 



to distal end 38 39 

 Posterior face of articular head to distal end of 



coracoid process 162 160 

 Greatest anteroposterior diameter of articular 



head 112 114 



Greatest transverse diameter of articular head 83 84 



