302 



TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA 



mulated a change in proportions and in a number of 

 details of cranial and dental structure which we may 

 collectively dignify by the term species, in the neo- 

 Linnaean sense. The transitional or intermediate 

 stages, distraguished by the infinitesimal advance in 

 certain new characters, are mutations in the sense of 

 that term as used by Waagen. The orthogenetic or 

 direct and adaptive origins of single new characters are 

 rectigradations in the sense of that term as used by 

 Osborn. (See p. 812.) The progressive changes in 

 certain characters — for example, in the rectigradations 

 of the premolar teeth and in the rise of the horn 

 rudiments on the frontals — occur nearly contemporane- 

 ously in members of the two phyla. In some other 

 characters the progression is dissimilar, or at different 

 rates. 



Geologic horizons of Limnohyops and Palaeosyops 



PROGRESSIVE DISTINCTIONS BETWEEN PALAEOSYOPS AND IIMNOHYOPS 



It is extremely difficult — indeed, it may be impossi- 

 ble — to distinguish parts of individuals belonging to 

 Palaeosyops from parts of those belonging to Limno- 

 hyops. Means of recognizing the differences and 

 resemblances have been afforded by the cumulative 

 work of Marsh, Earle, Osborn, and Gregory. 



The supposed distinctive generic character (Marsh) 

 of LimnoTiyops, namely, the presence of a hypocone on 

 m^, is possibly a primitive character, because of its 

 presence in Lamidotherium. It prevails but does not 

 appear to be constant in aU species of Limnohyops. 

 It is typically absent but exceptionally present, by 

 reversion perhaps, in Palaeosyops. In the proportions 

 of the skull Palaeosyops is more robust and Limno- 

 hyops is more slender, and this quantitative or pro- 

 portional contrast prevails throughout all the cranial, 

 dental, and skeletal parts, although it is often difficult 

 to measure or define the finer shades of difference. 



When we compare the ancestral members of the 

 two phyla in Bridger B, some of them are difficult to 

 separate. As the successive specific stages of Limno- 

 hyops are contemporaneous geologically with those of 

 Palaeosyops it is well to enumerate the chief known 

 distinctions which gradually develop and become fully 

 apparent only after the two lines of descent have 

 diverged from each other, as observed in the higher 

 geologic levels — for example, in comparing P. rolustus 

 and L. laticeps of Bridger D. These distinctions are 

 as follows: 



1. The upper and lower molar teeth of Palaeosyops 

 are relatively larger, more rounded, and more robust 

 than those of Limnohyops. 



2. The vertical striations on the cones of the upper 

 and especially of the lower molars of Palaeosyops are 

 more distinctly marked. 



3. On the upper molars (m'~^) of PaJaeosyops the 

 conules are more variable, more rounded, and sepa- 

 rate; in Limnohyops they are more constant, lophoid, 

 ridged, or conjoined with the protocone and hypo- 

 cone; this distinction, however, is not invariably 

 reliable. 



4. In m' of Limnohyops the hypocone is typically 

 though not invariably present (L. laticeps), and the 

 metaconule is extremely reduced, owing to the large 

 size of the adjacent hypocone. In m^ of Palaeosyops 

 the hypocone is typicaUy absent but sometimes pre- 

 sent, as in the type of P. diaconus. In m^ of Palaeosyops 

 the metaconule is generally present and in some 

 specimens is so close to the raised posterior cingulum 

 as to appear like a hypocone; thus the m' of Palaeo- 

 syops is generally more triangular, whereas that of 

 Limnohyops is more quadrate and sometimes actually 

 bUobed internally. 



5. The parastyle in Palaeosyops is rounded and 

 obliquely placed across the outer angle of the crown, 

 whereas in Limnohyops it is sharp and extends out- 

 ward as a ridge, analogous to the parastyle of the 

 Telmatherium type (Pis. LX, LXIII). 



6. The nasals taper toward the extremities and are 

 more pointed in Palaeosyops, whereas in Limnohyops 

 the sides of the nasals are more parallel and they are 

 more truncate at the extremities. 



7. The suborbital bar in the two genera becomes 

 quite different; in Palaeosyops the bar is roimded and 

 the overlying maxillary process extends back on its 

 outer side as a broad splint, whereas in Limnohyops 

 the bar becomes more depressed and slightly rectan- 

 gular in section and the maxillary process extends 

 back as a long, slender splint on the lower side; in 

 Telmatherium the suborbital bar is distinctly rectan- 

 giilar and the maxillary process extends back as a 

 long, narrow splint beneath the malar projection. 



8. In LimMohyops the top of the cranium is slightly 

 concave; in Palaeosyops there is a strong median con- 

 vexity near the frontoparietal junction some distance 

 behind the orbits. 



