MIOCENE PORPOISES — KELLOGG 149 



HYOID BONES 



The basihyal, the left thyrohyal, and the portions of the left stylohyal 

 were associated with pieces of the skull. These hyoid bones resemble 

 somewhat those of Delphinodon dividum (True, 1912, pi. 25, figs. 5, 18). 



The central basihyal (pi. 18, fig. 1) is almost hexagonal, rather thin 

 dorsoventrally, and dorsally exhibits a concave curvature from side 

 to side. The ventral surface is raised transversely to form a poorly 

 defined ridge. The ceratohyals were not preserved but were attached 

 to two short irregular projections on the anterior border. 



The rather broad and dorsoventrally compressed lateral wings 

 (thyrohyals) , which were not ankylosed to the basihyal, curve up- 

 ward and backward but are not bent downward distally. The left 

 thyrohyal (pi. 18, fig. 2) is thickened at the base where attached to 

 the basihyal, dilated medially, more or less flattened on the ventral 

 face, and tapers toward the extremity. 



The left stylohyal (pi. 18, fig. 3) is represented by five fragments, 

 which, when fitted together, indicate a length of 105 mm. It is 

 widened near the middle and curved slightly from end to end. The 

 anterior face is rounded and the posterior face is compressed, im- 

 parting a somewhat ovoidal shape to the stylohyal in cross section 

 near the middle of its length. The rugose, dorsoventrally compressed 

 distal end of the stylohyal normally fits into a depression on the 

 ventral border of the exoccipital, and the elliptical proximal end has 

 its long axis at right angles with the shaft of the bone. At 35 mm. 

 from the proximal end, the stylohyal was at least 12 mm. thick. 



Measurements of the hyoid bones (in millimeters) 



Greatest anteroposterior diameter of basihyal 45. 



Greatest transverse diameter of basihyal 56. 



Greatest length of left thyrohyal 81. 



Greatest width of left thyrohyal beyond base 28. 



Greatest length of stylohyal 105. ± 



Greatest width of stylohyal 19. 



VERTEBRAE 



Two vertebrae which are characterized by unusually high and 

 narrow neural canals were found associated with the mandibles and 

 other skeletal elements. One of them is an anterior dorsal vertebra 

 (pi. 19, fig. 1), possibly the fourth or the fifth. This vertebra differs 

 from the corresponding dorsal vertebrae of Delphinodon dividum in 

 having a relatively high and narrow neural canal and a relatively 

 longer centrum (length, 35 mm.) as compared to its width anteriorly 



338378—55- 



