822 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



form, ill which all parts are more or less imiformly 

 elongated or broadened together, there may also be 

 disharmonic (that is, differential) evolution of head 

 form, in which certain bony parts, such as the cranium, 

 are lengthened while others, such as the cheek bones, 

 are broadened. 



The continuous proportional evolution of the skull 

 of the titanotheres has been demonstrated by the 

 long investigations of the writer and his assistant. 

 Dr. W. K. Gregory, which involved thousands of 



parative anatomy and in the investigations made for 

 the present monograph.^' It is as follows: 



1. Elongation of the skull as a whole = dolichocephaly. 

 Elongation of the cranium only = doliohocrany. 



2. Broadening of the skull as a whole = brachycephaly. 

 Broadening of the cranium only = brachycrany. 



3. Elongation of the face only (that is, proopic dolichocephaly, 



as in £gM us) =dolichopy. 



4. Abbreviation of the face only = brachyopy. 



5. Elevation of the skull and head as a whole, as in Delphinus 

 = acrocephaly, hj-poicephaly. 



~7^roportion 



breadth increments 



T^ercentcufe 



zp /5 /o ^ 



rindinq teeth to foramen ovale \ — JDolickarhL — 



i-enyth of nasals 



T^a laeosyops 

 IDolichork in us 



T^espectiVe facial and cranial lengt/i of 7-hlaeosyops 



nespective premolar and molar len^iliofT-'alcLeosyops 



J)olichor}t I n.u.5 



AenctA increments 



S 20 2S 30 35 40 45 SO SS 60 65 70 75 



Figure 738. — Differential and distinctive] increment m every propoitional biocharacter group in eight bio- 

 characters of Palaeosyops and Dohchoihmiis 



measurements of specimens belonging to a number of 

 successive phyletic series, especially in Palaeosyops, 

 Megacerops, Menodus, and BrontotJierium. This con- 

 tinuity has been shown in the molar-cephalic index, 

 which is the breadth across the cheek arches X 100 -4- the 

 basilar length of the skull; also in other indices, such 

 as the faciocranial, in which the continuous trend of 

 proportional change has been carefully measured. It 

 has been demonstrated that elongation of the skull 

 as a whole (dolichocephaly) and broadening of the 

 skull (brachycephaly) may arise independently in 

 every phylum or line of descent. 



As shown in Figure 739, the titanotheres, like man, 

 exhibit facial abbreviation (brachyopy) and cranial 

 elongation (dolichocrany) in contrast to the facial 

 elongation (dolichopy) and cranial abbreviation (bra- 

 chycrany) of the horses. The changes of head form 

 in Dolichorhinus are shown to be continuous and to 

 result in dolichocephaly and cyptocephaly — that is, 

 bending down of the face upon the cranium, as in the 

 reindeer (Rangifer) and the hartebeest (Buhalis). 

 The adaptive mechanical significance or selection 

 survival value of these allometrons is sometimes 

 apparent, sometimes obscure. 



TERMINOLOGY INDICATING PROPORTIONS OF THE SKULL 

 AND SKELETON 



The general terminology of the changes in the pro- 

 portion of the skull and postcranial skeleton has been 

 gradually developed in the study of human and com- 



Th/aeosyoyps Manteoceras Da/i'cAorAinu^ 



Figure 739. — Continuous origin of allometron biochar- 

 acters in the cranium and skull of man (A) and of 

 the titanotheres (B) 

 In reference to man the words brachycephalic, mesaticephalic, and doli- 

 chocephalic denote respectively, brachycranial, mesaticranial, and 

 doliehocranial — in other words, they describe the proportions of the 

 cranial cavity. In reference to the titanotheres the same words are used 

 to describe the relative length and breadth of the entire skull. Note 

 especially the relative position of the orbit (o) . 



^8 The author's chief contributions to the craniometry of the titanotheres and 

 other ungulates are the following: Dolichocephaly and brachycephaly in the lower 

 mammals: Am. Mus. Nat. Hist. Bull., vol. 16, art. 7, pp. 77-89, Feb. 3, 1902; 

 Coincident evolution through rectigradation (third paper): Science, new ser., 

 vol. 27, No. 697, pp. 749-752, May 8, 1908; Skull measurements in man and 

 the hoofed mammals: Science, new ser„ vol. 35, No. 902, p. 596, Apr. 12, 1912; 

 Craniometry of the Equidae: Am. Mus. Nat. Hist. Mem., new ser., vol. 1, pt. 3, 

 pp. 57-100, flgs. 1-17, June, 1912. 



