UNITED STATES NATIONAL MUSEUM BULLETIN 294 



skull. Among Eecent odontocetes the most pronounced 

 backward extension of the ])alatines is obsei-vable on 

 zijjlioid skulls bolonginji: to the genera Berardtus, 

 Ziphin-^, and Mesoplodon. 



A gradual impixn-ement in the steei-ing function of 

 the forelimb is e\idenc«d in the trend toward shorten- 

 ing of the humerus according to Mchedlidze (1964, pp. 

 45-46), n. trend he thought api)eared earlier in the geo- 

 logical liistory of the cetotheres than in the Deljjhi- 

 noidea. The forelimbs are employed in turning, diving, 

 and balancing. 



Howell (1930, p. 231) mentioned the phenomenal 

 shortening of the humerus of Recent ceta^-eaiis and also 

 directe<l attention to the obsen'able variability in the 

 radial length. The humerus of many Recent odonto- 

 cetes is shorter than the radius, but there are exceptions 

 to this condition. The humeinis of Physeter, Kogia, 

 Pontoporin, and PJatanista is definitely longer than 

 the radius, the length of the hiunenis of the Ganges 

 river dolphin being twice the radial length. The sig- 

 nificance of tliis discordance in the relative lengths of 

 the humenis and radius ajnong living odontocetes is 

 not readily apparent. The humerus of at least four 

 Calvert Miocene por|X)ises, Squalodon calve rfen.s-is, 

 EurhinodeJphis bossi, Kentriodon pernlx, and Del- 

 fhiriodon divvdum,, is longer than the radius; the radius 

 of an adult Squalodon is about thi-ee-fourths of the 

 length of the humerus. The presumed original mobility 

 of the elbow joint of these four fossil porpoises has 

 been largely lost. There are unquestionably several 

 different factors, including the shifting of the origin 

 and insertion of the intrinsic musculatui-e, that have 

 accelerated and/or retarded the shortening of the 

 humerus in some cetaceans. 



The humerus of the North American arcliaeocetes 

 Basilosaurus cetoides and Zygorhiza kochii is elongated, 

 the length of the radius Ijeing eqtiivalent, respectively, 

 to one-half to seven-tenths of the length of the humerus. 

 Both of these archaeocetes have retained the ball and 

 socket character of the shoulder joint and the synovial 

 character of the ellx)w joint. 



Conversely, the humerus of the large Calvert ceto- 

 theres is shorter than the radius, being equivalent to 66 

 percent of the length of the radius of Pelocetufi cal- 

 vertensis, in contrast to a ratio of 74 percent (USNM 

 23058) and 77 percent (USNM 23059) for two other 

 large individuals from this formation, and at least 75 

 percent for the balaenoptcrid '^Exehriehttux" cephali/^s 

 Cope. The humerus of this Choptank cetothere is equiv- 

 alent to 67 percent of the lengtli of the radius. These 

 Miocene cetotheres have retained the ball and socket 



character of the shoulder joint, but have lost the syno- 

 vial character of the elbow joint. 



This shortening trend has not proceeded as far in 

 the slow swimming Balaenidae as in the balaenopterine 

 whales, the length of the humerus of both Balaena 

 mysticetw'i and Euhalaena gJaclalis being equivalent to 

 or at least nine-tenths of the length of the radius. The 

 pectoral flippers of these smooth-throated whales are 

 wide and stubby, and are equivalent to 12 to 18 percent 

 of length of whale. 



Shortening of the humerus seems unrelated to swim- 

 ming speed. The entire forelimb of the rather slow" 

 swimming humpback {Megaptera novaeangliae) is ex- 

 cessively elongated, occasionally one third of the whale's 

 length, and yet the relative lengths of the two upper 

 arm bones are not materially different from those of a 

 large-fin or furrow-throated whale {Balaenoptera) , the 

 length of whose humerus was equivalent to about 65 

 percent of the length of the radius. 



An efficient steering organ would serve a more useful 

 function for a rapid than for a slow swimmer. The sei 

 whale {Balaen-optera borealis) is considered to be the 

 fastest swimmer among living balaenopterids and is 

 reputed to attain a speed of 35 knots an hour for several 

 minutes. The length of the pectoral flipper, tip to axilla, 

 of the adult sei whale is equivalent to 8 to 11 percent of 

 the total length of the whale, which compares favorably 

 with 8 to 10 percent for the finback and 10 to I21/2 per- 

 cent for the blue whale. Blue and finbacks have been 

 observed to maintain a speed of 18 to 20 knots for 10 

 or 15 minutes, their nonnal cruising speed being 10 

 to 12 knots. The length ratio of the humerus to the 

 radius of one 45-foot adult sei male is 49 percent. For 

 other living balaenopterid whales, however, especially 

 BalaenojHera acutorontrata and B. physahis, this length 

 ratio is as low as 58 and as high as 70 percent. 



There is, thus, no evidence of a direct relationship 

 between either the overall length of the pectoral flipper 

 or the length ratio of liumerus to radius and the cruising 

 speed of the whale. Until skeletons of Oligocene or 

 geologically older predecessors of the middle Miocene 

 cetotheres are discovered, further speculation may not 

 prove very informative. 



Thinocetus,' new genus 



Type-species. — Thinocetus arthritus, new species. 



I>iAGNOsis. — Atlas massive, thick; a short rugose, 

 transversely widened hyapophysial process present. 

 Odontoid process of axis slender, unusually elongated. 



' Thinos, .shore, in allusion to the presumed stranding of this 

 mysticete on the shore. 



