CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



827 



cephalic) and highly differential rates of evolution in 

 different parts of its skull and teeth. Although 

 progressive brachycephaly is the chief harmonic trend 

 of development, this harmonic trend affects every bone 

 in different degree. 



DOLICHORHINUS: ADAPTATION OF THE LENGTHENED 

 HEAD TO THE SUPPOSED HABIT OF GRAZING 



In the Dolichorhinus phylum (Dolichorhininae) we 

 are afforded a unique opportunity of studying the 

 allometry that transforms a mesaticephalic skull 

 {Mesatirhinus peter soni) into a dolichocephalic skull 

 (DolicJiorJiinus hyognathus) with a strong tendency 

 to cyptocephaly. The comparison between the older 

 and newer forms is not absolutely exact because we 

 can not yet demonstrate positively that Dolicho- 

 rhinus was directly descended from Mesatirhinus, but 

 we are certain that the changes came about through 

 a general lengthening of the skull with a relative 

 narrowing; moreover, that this is the result of a 

 differential evolution in which some bones grow much 

 more rapidly than others. At least part of these 

 changes are apparently correlated with adaptation 

 to the habit of grazing. 



A comparison of Dolichorhinus and Palaeosyops dis- 

 closes the following eight features of Dolichorhinus: 

 (1) There is a general dolichocephalic increment of 

 33 per cent; (2) harmonic increment is rare; (3) incre- 

 ment in length greatly exceeds increment in breadth; 

 (4) cranial increment (dolichocrany) exceeds facial 

 increment (dolichopy); (5) the increment in breadth is 

 the minimum; (6) head bending (cyptocephaly) is due 

 to the increment in the length of the roof bones of 

 the skull in excess of those of the basicranial line; 

 (7) the increment of the grinding series and of the 

 cranium is harmonic; (8) the increment of different 

 members of the grinding series is differential, dis- 

 harmonic. 



DOLICHOCEPHALIC INCREMENTS OF MESATIRHINUS AND 

 DOLICHORHINUS 



As Mesatirhinus petersoni is itself a dolichocephalic 

 form these changes are not so marked as they would 

 be were we to compare Dolichorhinus hyognathus with 

 its unknown mesaticephalic ancestors, yet we discover 

 very marked allometry. Partly owing to the crushed 

 condition of the skulls a very high degree of accuracy 

 can not be claimed for the percentages in the table, 

 although they are based on averages obtained from 

 a very large number of measurements. The two ghief 

 peculiarities of Dolichorhinus hyognathus as compared 

 with Mesatirhinus petersoni are the excessive length 

 of the skull as a whole and the convexity (cypto- 

 cephaly) of the cranium. 



The cyptocephaly of the skull was brought about 

 by more rapid evolution of the top or upper bones 

 than of the bottom, the basicranial and palatal bones. 



The total skull measurement (cranium to face) from 

 the tip of the nasals to the tip of the occiput along a 

 dorsal line increased 44 per cent, whereas the measure- 

 ment along the inferior line, premaxillaries to condyles, 

 increased only 33 per cent. The top of the cranium 

 proper (postoi-bital process, frontals to tip of occiput) 

 shows an increase of 40 per cent, whereas the cor- 

 responding measurement of the basal line (original 

 boundary of posterior nares to posterior face of 

 condyles) shows an increase of only 30 per cent. 

 The basis of the cranium, the cranio-facial angle, has 

 been produced downward with relation to the basis 

 of the face from 157° (or 23°) in if. petersoni and 136° 

 (or 44°) in D. hyognathus — that is, through 21°. 



The skull as a whole, premaxillaries to condyles, 

 and the cheek teeth as a whole, p'-m^, increased 

 about equally in length (33 per cent). A similar 

 equal increase of the skull and cheek teeth also marks 

 the progressive dolichocephaly of the Oligocene genus 

 Menodus. The cranium lengthens more rapidly than 

 the face. The proportional evolution of the cranium 

 and face can best be measured on a horizontal line 

 from the tip of the premaxillary to the condyle, 

 by projecting on this line the tip of the postorbital 

 process of the frontal. The face increment (26 per cent) 

 is thus seen to be 12 per cent less than the cranium 

 increment (38 per cent) and 7 per cent less than the 

 increment of the skull as a whole (33 per cent). The 

 region of the greatest elongation of the cranium is 

 that between the hinder border of m^ and the foramen 

 ovale, which increases 54 per cent. In front of and 

 behind this region the increments in length are less, 

 as shown in the series of detailed comparative measure- 

 ments below. It is in this greatly stretched region 

 that many anatomical peculiarities, such as the 

 extremely elongate posterior nares and the protrusion 

 of the ethmoturbinal, the secondary palate, are noted. 



Other features of differential evolution in length 

 are as follows: The premaxillaries and the postcanine 

 diastemata have lengthened faster (increment 44 per 

 cent) than the skull as a whole (33 per cent). The 

 free portion of the nasals has increased 55 per cent, 

 and the nasals as a whole have increased 70 per cent 

 as compared with the skull as a whole (33 per cent). 

 The zygomata have increased 44 per cent, or much 

 more than the skull as a whole (33 per cent). There 

 is a marked forward displacement of the postglenoid 

 process, the distance between the postglenoid and 

 post-tympanic processes having increased 55 per cent, 

 whereas the corresponding distance between the 

 foramen ovale and foramen condylare has increased 

 only 34 per cent. As a result of this forward dis- 

 placement of the postglenoid process, the external 

 auditory meatus has been rotated forward from an 

 angle of 78° in M. petersoni to a corresponding angle 

 of 53° in D. hyognathus. The anteroposterior diameter 

 or opening of the external auditory meatus has 



