826 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Differential measurements, in millimeters, in seventeen dental and cranial characters oj shulls transitional between 



Palaeosyops major, P. leidyi, and P. rohustus 



[All from the Bridger formatiou] 



On account of the breaks in the phyletic series and 

 the effects of distortion the results presented in the 

 above Palaeosyops table are approximations. They 

 would have been much more strddng had it been 

 possible to compare completely the earliest, smallest 

 known form, P. paludosus, with one of the latest and 

 largest forms, P. grangeri, and had all the skulls been 

 free from distortion. In this table are compared only 

 the three intermediate stages, P. major, P. leidyi, 

 P. rohustus, from which the following principal 

 generalizations may be made. 



HARMONIC ALLOMETRY EXCEPTIONAL 



Of the 17 characters mentioned in the above table 

 2 only are harmonic, giving exactly the same per- 

 centages of increase. Other characters which show 

 near approximations to harmonic increase are (4) 

 the length of the skull, (2) the length of the grinding 

 teeth, (1) the breadth of the fourth premolar, (10) 

 the width of the occipital condyle. These nearly 

 harmonic increments range from 11 to 14 per cent. 



The skull increases 7 per cent faster in breadth than 

 in length, the breadth increment being 1.6 times the 



length increment. From P. major to P. robustus 

 the skull increases in length 50 millimeters, or 11 

 per cent; it increases in breadth 57 millimeters, or 

 18 per cent. 



Cranial and facial increments are subequal. The 

 gain in the length of the preorbital region is 12 per 

 cent; the gain in the postorbital region is 13 per cent. 

 Thus in Palaeosyops the face and the cranium increase 

 at approximately equal rates, whereas in most other 

 titanotheres the cranium increases more rapidly than 

 the face. For example, in DolicJiorJiinus (as compared 

 with MesatirJiinus) during a given period the face 

 increases 26 per cent and the cranium 38 per cent. 



The greatest gain differentially is in the breadth of 

 the masseteric muscle area of the zygomata. On 

 comparing a young adult specimen of P. major with 

 an aged specimen of P. rohustus we observe a gain of 

 73 per cent in the depth of the malar below the orbit. 



When we compare these percentages with those 

 based on similar measurements of skulls of specimens 

 of other genera we again establish as the distinct 

 generic character of Palaeosyops that it is a genus 

 characterized by certain definite (that is, brachy- 



