520 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



the skull almost equals the giant Brontoiherium, but 

 in width across the zygomata it is far inferior to this 

 animal; in the length of its grinding series it is far 

 superior to either Brontops or Brontotherium; in the 

 ratio of its grinding series to the entire length of the 

 skull it is again superior, as well as in the actual 

 measurements both of its premolar and of its molar 

 series. Its fourth premolar is actually longer and 

 narrower than that of Brontotherium. The sum of the 

 transverse measurements of its grinding teeth is 

 decidedly less than that in Brontops or BrontotJierium. 



The table on page 523 shows that dolichocephaly, a 

 distinctive phyletic character of Menodus, is impressed 

 not only upon the skull but upon the teeth. 



The horns preserve their triangular or trihedral 

 shape because the connecting crest as seen in the 

 upper view of the type of Menodus giganteus (Pi. 

 CXXXVIII, fig. 391) is strongly developed, as well as 

 the anterior ridge which extends downward into the 

 sides of the nasals and the external ridge, the latter 

 extending laterally into the antorbital bar. In the 

 beautifully preserved male sicull from the summit of 

 the Titanotherium zone in the Field Museum (PI. 

 CXL) the pointed apices of the horns are clearly 

 displayed. As observed in the side view of the same 

 skull (PI. CXXXIX) the facial or preorbital portion 

 of the skull is relatively elongate, and there is a broad 

 bridge across the infraorbital foramen as well as a 

 very deep anterior junction of the premaxillaries. 

 The zygomatic arches are strengthened by depth 

 rather than by breadth; it is true that a moderate 

 expansion is observed in this and other old males, 

 such as the type of T. "ingens," but the enormous 

 buccal swellings characteristic of Brontops and Bron- 

 totherium are not developed. In the auditory region 

 a highly dolichocephalic character appears — namely, 

 the relatively open condition of the external auditory 

 meatus due to the noncontact or retarded contact 

 of the postglenoid and post-tympanic processes. 

 Similarly, the occipital condyles project widely at 

 the back of the skull. The occiput is relatively high 

 and ascends rapidly above the condyles. The jaw is 

 also highly distinctive, with its long, straight lower 

 border, its backward rather than downward project- 

 ing angle, and its well-defined chin. It is totally 

 different from the jaw of Brontotherium. 



In Menodus giganteus the dental series attains the 

 finest proportions known in any member of the order 

 Perissodactyla. The vigorous development of the 

 teeth, exhibited in the large canines, in the pronounced 

 cingula, in the length of the grinding series as a whole, 

 in the height of the crown, in the excess of anteropos- 

 terior over transverse measurements of the grinders, 

 and in the progressive dental index, affords one of the 

 most distinctive characters of this phylum as a whole. 



Harmonic evolution. — In progressive increase in size 

 all parts of the skull and dentition share alike between 



the stages M. heloceras and M. giganteus — namely, 

 about 50 per cent. The horns increase in length 190 

 per cent as compared with an increase of 250 per cent 

 during the same period in Brontotherium — that is, they 

 somewhat more than share the general increase in 

 length of the skull, but they do not dominate in de- 

 velopment to the detriment of other features as in 

 Brontotherium. 



Sexual characters. — The differences between the 

 males and females are very marked, as clearly shown 

 in the contrast between two male and two female 

 skulls in the American Museum. The female skulls 

 are smaller in most of their dimensions, less rugose on 

 the surfaces for muscular attachment. In the males 

 the horns are more robust, more decidedly triangular 

 rather than rounded, the triangular form being sharply 

 defined especially at the base. In their vigorous 

 growth they sometimes exhibit the anterior tuberous 

 branching, as seen in skull Am. Mus. 505. In the 

 females the horns are relatively slender, with less 

 anteroposterior diameter at the base, and more pointed 

 at the summit. While in both sexes the incisors are 

 vestigial and certainly do not pierce the gum, there 

 is marked disparity in the canines. In the males 

 (PL CXL) the canines are formidable weapons, the 

 anterior enamel face of one specimen (Am. Mus. 505) 

 being estimated at 70 millimeters in length, whereas 

 in females the canines are far more slender, the an- 

 terior face being 45 millimeters. On the grinding 

 teeth the cingulum is equally robust in both sexes — • 

 in fact, the most pronounced development of this 

 structure is observed in the female skull (Am. Mus. 

 1067), which is evidence that development of the cin- 

 gulum is not a sexual character. 



Progressive specific stages. — Three species are defi- 

 nitely distinguishable as characteristic of the lower, 

 middle, and upper zones respectively. There are 

 also two transition species and a closely connected 

 series of "ascending mutations." (See table on p. 523.) 



Menodus heloceras (Cope) of Chadron A: The type 

 of M. heloceras (Cope) was at first placed as the ances- 

 tor of this phylum, but with some hesitation because 

 of its imperfect preservation. So far as known it 

 conforms in skull structure, although in a very prim- 

 itive stage of development. Its ancestral position 

 in the Menodus phylum was confirmed (1910) by 

 Granger's discovery of a second skull at the base of 

 the Titanotherium zone. 



Menodus {" Symborodon") torvus (Cope) is an inter- 

 mediate stage between M. heloceras and M. trigonoceras ; 

 it is known only from the type jaw, which is the 

 genotype of the genus Symhorodon. 



Menodus {"Titanotherium") proutii Leidy: Leidy 

 founded the genus Titanotherium on two specimens, 

 the first was Pomel's type of Menodus giganteus; the 

 second specimen, which Leidy fully described and 

 measured, becomes the type of his species proutii; 

 it belongs to an animal intermediate in size between 

 M. torvus and M. trigonoceras. 



