816 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



the osseous horn, at the junction of the f rentals and 

 parietals (PL XLV, C); (5) shifting of the frontals and 

 the osseous horns to the back of the cranium through 

 allometry; (6) internal secretions from the interstitial 

 cells of the male (testis) or female (ovary) germ 

 glands. 



the junction of the frontals and parietals, the shifting 

 of the osseous horns forward from above the eyes, and 

 the changes in size and proportions, described in Chap- 

 ter V, section 2, and Chapter X, section 3. 



The horn rudiments in different phyla, like the cusp 

 rectigradations, show marked differences in phylo- 



FiGTJRE 734. — Independent appearance of rudiments 

 though in the same part of the skull, 



PHYLOGENESIS AND INITIATION OF HORNS IN 

 TITANOTHERES °* 



In contrast to the development of the horns of cattle 

 all that we paleontologists observe in the evolution of 

 the horns of the titanotheres is the rudimentary ap- 

 pearance of the bony swelling of the osseous horns at 



s' See dctafled descriptions in Chapter V. 



of the horn (H) at diflferent stages of geologic time, 

 in different phyla of the titanotheres 



genetic and ontogenetic velocity. One of the most sig- 

 nificant discoveries made concerning the titanotheres 

 is that their horn rudiments arise long after the phyla 

 have been separated from one another, earlier in some 

 phyla and later in others, but always on substantially 

 the same part of the skull. This is probably true in 

 general of the origin of horns in mammalian families. 

 Each family probably has its predisposition to develop 



