DISCOVERY OF THE TITANOTHERES AND ORIGINAL DESCRIPTIONS 



147 



formula |^, in the synopsis given by Professor Marsh (1876.1, 

 p. 339), as distinguished from Menodus, with ?pm J^. One 

 of the lower jaws of the Princeton collection, however, has the 

 premolar formula 57^, demonstrating that the first lower pre- 

 molar is a variable tooth and can not in this case be used in 

 classification. The same rule applies to the second cone upon 

 the last upper molar, the supposed generic character of Dicono- 

 don Marsh. This is found in different species in all degrees 

 of development, from a small prominence upon the basal cin- 

 gulum to a well-developed cone {M. proutii) . 



From this evidence Osborn draws the following 

 partly erroneous conclusion: 



Such characters as the invariable absence of lower incisors 

 may subsequently be found to separate one genus of the Meno- 

 dontidae from another; but our present evidence goes to show 

 that they simply characterize the extremes of a closely related 

 series of animals, from the same horizon, of which the inter- 

 mediate forms are represented by numerous species. The 

 safest basis of specific determination seems to be the correla- 

 tion between the developnaent and proportion of the horns and 

 of the nasals, the rule being that where the horns are long the 

 nasals are short, and conversely. The number of the teeth 

 does not at present seem to be absolutely constant, even within 

 the limits of the species. 



The following determination of the species in the Cambridge 

 collection is, for the above and other obvious reasons, provi- 

 sional. The classification can be finally settled only when the 

 lower jaws and skulls are found in association. 



Thus the validity of the several genera recognized 

 by Marsh and Cope and of the chief criteria used by 

 them as generic characters was called in question. 

 The species are treated as belonging mainly to the 

 single genus Menodus. Taking up the description of 

 the new material, the authors mistakenly refer to 

 Leidy's Megacerops coloradensis, a well-preserved 

 skull, which at present is referred to Alhps marshi. 

 They then describe two new species — "Menodus" 

 tichoceras, based on a skull, and "Menodus" platyceras, 

 based on a pair of bony horns. Both these species 

 are at present referred to the true Brontotherium or 

 flat-horned genus. The authors conclude their dis- 

 cussion of the "Menodontidae" by presenting the 

 first published restoration of the skeleton, made up 

 of material in several museums, forming a composite 

 animal representing Menodus proutii. In connection 

 with a table of measurements arranged to show pro- 

 gressive and correlated changes in the horns and nasals, 

 they make the following remarks (op. cit., p. 16): 



The above measurements bring out very clearly the decrease 

 in the proportions of the nasals pari passu with the gradual 

 elongation of the horns. Another very interesting fact is 

 brought out by the comparison of the transverse and longitudi- 

 nal diameters of the horns at the base. As we pass from the 

 short to the long horned types, through M. coloradensis, ticho- 

 ceras, doUchoceras, and platyceras, there is a gradual rotation of 

 the longer axis of the horn section from a fore and aft to a trans- 

 verse plane, the species last named representing the extreme of 

 the transverse type. 



The fuller development and more or less radical 

 modification of the hypotheses put forward in this 

 paper have been the subject of successive contribu- 

 tions by Osborn, culminating in the present work. 



GEOLOGIC lEVEIS AND SUCCESSION OF TYPES (HATCHEE, 1886-1893) 



The work of Marsh and Cope had been exclusively 

 descriptive and systematic. Osborn's observation of 

 the correlated progressive reduction of the nasals 

 and the enlargement and flattening of the horns 

 seems to have been the first important application of 

 evolutionary principles to tlie study of the Oligocene 

 titanotheres. But materials for an exact knowledge 

 of the phyletic succession, resting securely upon a 

 knowledge of the precise geologic levels of a large 

 series of specimens, had hitherto been entirely lacking. 

 This all-important element of the time relations of 

 the different species was largely supplied by the 

 labors and study of J. B. Hatcher. In 1886, 1887, and 

 1888 Hatcher spent 15 months in the White River 

 beds of South Dakota and Nebraska, collecting 

 material for Professor Marsh's monograph on the 

 Titanotheridae. In an interesting article in the 

 American Naturalist for March, 1893, Hatcher (1893.1, 

 pp. 214, 215) tells us that he collected or purchased 

 "nearly 200 complete skulls and many more or less 

 complete skeletons," a part of which are now on 

 exhibition in the National and Yale Museums. The 

 superb Hatcher collection in the United States 

 National Museum contains skulls and jaws of 157 

 individuals; as completely listed in the generic 

 sections of this monograph, it furnishes the classic 

 standard of reference. Hatcher writes: 



Early in the season of 1886 it became apparent that certain 

 forms of skulls were characteristic of certain horizons in the 

 beds. This fact showed the importance of keeping, so far as 

 possible, an exact record of the horizon from which each skull 

 or skeleton was taken. From actual measurement the vertical 

 range of the Titanotheridae was found to be about 180 feet. 

 For convenience in keeping a record of horizons the beds were 

 divided into three divisions of 60 feet each, and each of these 

 three divisions was subdivided into three divisions of 20 feet 

 each. The difi'erent skulls and skeletons, when dug out, were 

 each given a separate letter or number, and this letter or num- 

 ber was placed in that subdivision of the beds from which the 

 skull or skeleton was taken. 



At present about 60 of these skulls and several more or less 

 complete skeletons have been freed from their matrix. When 

 studied in connection with the horizons from which they were 

 taken, these remains show that a regular and systematic 

 development took place in these animals from the base to the 

 top of the beds. The most noticeable change which took place 

 in the Titanotheridae was a gradual and decided increase in 

 their size from the lowest to the uppermost beds, as is shown 

 by the increase in the size of the skulls, fore and hind limbs, and 

 other portions of the skeleton. Individuals found near the 

 bottom of the beds are little, if any, larger than the living 

 rhinoceros. From this they gradually increase in size as we go 

 up until at the top we find a type described by Professor Marsh 

 as Titanops, rivaling the modern elephant in size. 



This increase in size from the base to the summit of the beds 

 was attended by a very marked development in certain portions 

 of the skeleton, noticeable among which are the following: 

 A variation in shape and an increase in the size and length of 

 the horn cores as compared with the size of the skulls, attended, 

 near the summit of the beds at least, by a decided shortening 

 of the nasals. 



