CETOTHERES FROM THE MIOCENE CHOPTAISTK FORMATION 



above and in front of the anterointernal rim of the 

 entrance to the mandibular canal. 



The condyle (106X164 mm.; fig. 2) of the type left 

 mandible is large, expanded transversely in an oblique 

 direction above the middle of its vertical diameter and 

 is more convex from side to side than dorsoventrally. 

 Above the protuberant and convex angle, the deep 

 groove for the attachment of the internal pterygoid 

 muscle tends to impart a contour to the internal edge 

 similar to the external edge of the condylar articular 

 surface. Dorsally the attenuated condylar articular sur- 

 face merges anteriorly with the curved thin rim of the 

 horizontal ramus behind the coi-onoid process, and al- 

 though narrowed is actually more noticeably trans- 

 versely thickened than on Calvert mandibles of similar 

 length. The forward curving internal and external bor- 

 ders of the condyle project noticeably beyond the lateral 

 surface of the adjacent portions of the ramus. One may 

 assume that this condyle was enveloped by a thick fi- 

 brous pad, similar to that of Recent mysticetes (Turner, 

 1892, p. 69), which served as the means of attachment 

 to the glenoid fossa of the zygomatic process. 



Table 4. — Measurements {in mm.) of left mandibles 



Figure 2. — Posterior view of con- 

 dyle of left mandible, USNM 

 23794, of Thinocetuj arthritus. 

 an, angle; cm., condyle of man- 

 dible; i.pt., groove for internal 

 pterygoid muscle. 



I.pt. 



Of the several mysticete mandibles recovered during 

 the excavations in the Antwerp basin, this Choptank 

 mandible exhibits the closest resemblance to Mesocetus 

 pinguk (Van Beneden, 1886, pi. 44, fig. 10; right con- 

 dyle, 96.5X164 mm.), except that the angular portion 

 of the condylar articular surface below the inner groove 

 for attachment of the internal pterygoid muscle is nar- 

 rower and prolonged farther downward. The vertical 

 diameter (123 mm.) of the anterior end of the right 

 mandible of M. pinguLt (MHNB 13) is greater than the 

 corresponding measurement (108 mm.) of this Chop- 

 tank mandible, but is narrower (39.5 mm.) in this region 

 than the latter (43 mm.). Proportional growth rates of 

 portions of the mandibular ramus of these fossil mysti- 

 cetes are not as yet definable. 



See table 4 for measurements of the mandibles. 



Greatest length In a straight line, as preserved 



Greatest vertical diameter 100 mm. behind 

 anterior end of ramus 



Greatest transverse diameter 100 mm. behind 

 anterior end of ramus 



Greatest vertical diameter 1350 mm. anterior 

 to posterior face of condyle (300 mm. be- 

 hind anterior end of ramus) 



Greatest transverse diameter 1350 mm. an- 

 terior to posterior face of condyle (300 mm. 

 behind anterior end of ramus) 



Greatest vertical diameter 1150 mm. anterior 

 to posterior face of condyle (500 mm. be- 

 hind anterior end of ramus) 



Greatest transverse diameter 1150 mm. an- 

 terior to posterior face of condyle (500 mm. 

 behind anterior end of ramus) 



Greatest vertical diameter 950 mm. anterior 

 to posterior face of condyle (700 mm. behind 

 anterior end of ramus) 



Greatest transverse diameter 950 mm. anterior 

 to posterior face of condyle (700 mm. behind 

 anterior end of ramus) 



Greatest vertical diameter 650 mm. anterior to 

 posterior face of condyle (1000 mm. behind 

 anterior end of ramus) 



Greatest transverse diameter 650 mm. anterior 

 to posterior face of condyle (1000 mm. be- 

 hind anterior end of ramus) 



Greatest vertical diameter through coronoid 

 process 



Horizontal distance from center of coronoid 

 process to posterior face of condyle 



Greatest vertical diameter of posterior end 

 including condyle 



Greatest transverse diameter of condyle 



VSNM USNM 

 SS794 $3961 

 1575-t- 1285-1- 



108 

 43 



95 



63 



106 



71 



164 



240 



165 

 106 



99 

 46 



92 



55 



93 



64 



97 



72 



FORELIMB 



The right forelimb is represented by the scapula, radius, 

 ulna, six carpals, four metacarpals, and four phalanges. 

 An incomplete scapula, humerus, ulna, one metacarpal 

 and one phalange belong to the left forelimb. Assuming 

 that the numlwr of phalanges in the longest finger was 

 seven, similar to Balaenoptera aciitorostrata, the length 

 of the manus would be approximately 450 mm. and the 

 skeletal length of the fore flipper from head of humerus 

 to distal end of terminal phalange about lOTO mm. (42 

 inches). The distance from the vertebral margin of the 

 scapula to the distal end of the radius is 890 mm. (about 

 35 inches). The length of the humerus (258 mm.) is 

 67 percent of the length of the radius (385 mm.). The 

 length of the fore flipper, tip to axilla, is estimated to 



