CETOTHERES FROM THE MIOCENE CHOPTANK FORMATION 



HUMERUS RADIUS 



Both humeri (USNM 23794) are well preserved and 

 retain both epiphyses firmly ankylosed to the shaft. 

 Although the length of the right humerus of this Chop- 

 tank mysticete is nearly equivalent to that of Pelocetus 

 calvertensis (USNM 11976 ; 266.5 mm.) it is less massive 

 as regards other measurements. Anteriorly, the large, 

 smooth, and convex head (pi. 3, fig. 2) is elevated above 

 the greater or radial tuberosity ; on the right humerus 

 the head is also separated from this tuberosity by a well- 

 defined groove in contrast to the himierus of P. calver- 

 tensis. The head projects outward and backward. In 

 Recent mysticetes the rugose greater tuberosity, which 

 projects internally, serves as the proximal area for the 

 attachment of the deltoid muscle. There is a large, irreg- 

 ular nodular tuberosity centrally placed on the intei-nal 

 face below and between the head and the greater 

 tuberosity. 



The shaft is more noticeably compressed transversely 

 than that of P. calvertensis. An elongated welt-like en- 

 largment (length, 115 mm. ; pi. 3, fig. 2) continuous with 

 the greater tuberosity is developed on the anterior or 

 radial face of the shaft. A narrower welt commencing 

 at the distal ulnar facet extends upwai'd along the inner 

 border of the hinder or ulnar face toward the head and 

 coalesces proximally with the above-mentioned median 

 proximal tuberosity on the right humerus; crushing has 

 obliterated this welt on the dorsal portion of the shaft 

 of the left humerus. The transversely flattened radial 

 facet is set off by a ridge-like crest from the more convex 

 transverse surface of the ulnar facet which is extended 

 upward on the posterior face of the shaft. 



See table 6 for measurements of the humerus. 



Table 6. — Measurements (in mm.) oj the humerus, USNM S3794 



Greatest length of humerus 



Greatest anteroposterior diameter of proximal end 

 Greatest anteropostericr diameter of head 

 Greatest exterointernal (transverse) diameter of 



head 

 Least anteroposterior diameter of shaft 

 Least exterointernal (transverse) diameter of shaft 

 Greatest anteroposterior diameter of distal end 

 Greatest exterointernal (transverse) diameter of 



distal end 

 Greatest anteroposterior diameter of radial facet 

 Greatest anteroposterior diameter of ulnar facet 



(in a straight line) 



The right radius (USNM 23794; pi. 3, fig. 3) has the 

 distal epiphysis detached from the shaft, but it was 

 preserved with the associated carpal bones. This distal 

 epiphysis measures 72X45 mm. and has a maximum 

 thickness of 23 mm. The right radius is slightly shorter 

 than that of P. calvertensis (USNM 11976; length, 397 

 mm.). This forelimb bone of the Choptank mysticete 

 is rather stout, slightly curved, and transversely com- 

 pressed, measuring 385 mm. in length. The proximal 

 facet, which articulated with the radial facet on the 

 distal end of the humerus, is shallowly concave; the 

 articular surface of this facet rolls over on the internal 

 face of the proximal end of the shaft. The facet on the 

 posterior face of the proximal end of the shaft for 

 articulation with the olecranon of the ulna is semicir- 

 cular in outline, its transverse diameter being 49 mm. 

 and its dorsoventral diameter 32 mm. 



The outer surface of the shaft is slightly more convex 

 than the internal face. The distal lialf of the posterior 

 edge of the shaft is more compressed tlian the anterior 

 edge. The deeply concave distal end of the shaft is pit- 

 ted for attachment of the cartilaginous connection with 

 the epiphysis. 



See table 7 for measurements of the radius. 



Table 7. — Measurements (in mm.) of the right radius, 

 USNM 23794 



Greatest length (lacking distal epiphysis) 

 Greatest anteroposterior diameter of proximal end 

 Greatest transverse diameter of proximal end 

 Least anteroposterior diameter near middle of shaft 

 Least transverse diameter near middle of shaft 

 Greatest anteroposterior diameter of distal end 

 Greatest transverse diameter of distal end 



385 

 81 

 56 

 76 

 33 

 88 

 42 



77 



75 



ULNA 



Both ulnae (USNM 23794) were associated with this 

 skeleton ; the distal epiphysis was detached from each 

 shaft. The proximal end of the ulna (pi. 3, fig. 4) is 

 expanded to form the somewhat hatchet-like olecranon 

 process which has a rugose straight vertical posterior 

 edge. The articular surface of the greater sigmoid cav- 

 ity, wli it'll limits movement of the distal ulnar facet of 

 the humerus, is truncated horizontally on its elevated 

 dorsal edge (width, 36 mm.) and is expanded ventrally 



321-614 0—69- 



