EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 



355 



condition of the premolars and in the cupping of the 

 incisors. It resembles MetarTiinus, especially M. 

 earlei, in the following characters: (1) Broad forehead; 

 (2) concavity of the face in top view; (3) certain fea- 

 tures of the premolars; (4) proportions of the occiput; 

 (5) thin, high sagittal crest. 



From the contemporary species of Manteoceras, 

 namely, M. uintensis, it is distinguished by (1) the 

 characters of the incisors and canines; (2) the much 

 more advanced condition of the premolars; (3) the less 

 elongate m^; (4) the feebly constricted postcanine 

 region. It parallels Manteoceras in the general pro- 

 portions of the skull and in the form of the zygomatic 

 arches, except that the malar portion of the arch has 

 the deep flange characteristic of Telmatlierium. 



Sthenodectes suggests Tehnatherium ultimum in cer- 

 tain features of the incisors, canines, and molars, in 

 the detailed characters of the basicranial region and 

 in the spreading zygomata; but it is distinguished from 

 that form by (1) the much larger size and higher 

 development of the incisors, (2) the more advanced 

 condition of the premolars and premolar cingula, (3) 

 the different form of m', (4) the lower occiput and 

 sharper sagittal crest, (5) the wider forehead, (6) the 

 more angulate section of the infraorbital portion of the 

 malars, (7) the sharply tapering nasal bones. From 

 the European genus Brachydiastematherium, which it 

 resembles in having three large incisors, Sthenodectes 

 is distinguished by the markedly lower evolution stage 

 of the premolars (p2-p4). 



Side and top views. — The top of the type skull has 

 been crushed downwai'd, especially above and in 

 front of the orbits. The premaxillaries, though some- 

 what flattened by pressure, are of very large size, in 

 correlation with the exceptional dimensions of the 

 incisors. The nasals are imperfectly preserved at 

 the end but appear to be even shorter than in T. 

 ultimum; they converge rapidly in front, about as in 

 MetarTiinus, and proximally they spread rapidly and 

 widely, measuring 125 millimeters transversely at the 

 outer junction with the frontals. The latter were 

 somewhat flattened but were very wide across the 

 orbits (tr. 192 mm.). In front of the orbits there is a 

 prominent vertical facial concavity suggesting the 

 conditions in MetarMnus fluviatilis. The infraorbital 

 foramen is large and prominent, apparently more so 

 than in T. ultimum. Above this foramen and in 

 front of the orbit is a triangular depression, in the 

 position of the lacrimal bone, occurring on both sides 

 of the skull but much larger in the right, which is 

 referred to by Douglass as a vacuity. It now seems 

 probable that these vacuities resulted from the down- 

 ward crushing which has squeezed the lacrimals out 

 of place; they lie immediately below the region where 

 the horn swelling usually appears, but the presence of 

 the latter is but vaguely if at all indicated. The fore- 

 head, as already stated, is broad and flat, and the depth 



of the skull appears to be less than in T. ultimum 

 The opposite postorbital temporal crests run backward 

 into a long sagittal crest, which is quite high and thin. 

 The occipital crests are thin, but the whole occiput is 

 much lower than in T. ultimum. 



Palatal view. — In the inferior aspect of the skull we 

 are struck by the great size of the dentition as a whole, 

 the great size and spatulate outline of the incisor re- 

 gion, the prominent pointed canine tusks, the long, 

 straight tooth row, the virtual lack of a postcanine 

 diastema, the wide, very progressive premolars, the 

 relatively large, subhypsodont molars, the widely 

 arching zygomata, and the short basicranial region — 

 all these, with the exception of the prominence of the 

 incisors and canines, being characteristic of Oligocene 

 titanotheres. The infraorbital part of the malar is 

 like that of Manteoceras in that it did not flare out- 

 ward into an infraorbital protuberance; just behind 

 the orbit the malar was very massive, and its broad 

 inferior expansion shows an area for the attachment 

 of the masseter; the postero-inferior portion of the 

 malar is a deep vertical flange, as in Manteoceras and 

 T. ultimum. The squamosal portion of the zygoma 

 is very stout and broad anteroposteriorly; the post- 

 glenoid process is rather small. The prominent ex- 

 ternal auditory meatus of the type in side view appears 

 to be closed below by the appression of the postglenoid 

 and post-tympanic processes, but this is probably due 

 to crushing, as the Field Museum specimen shows these 

 processes widely separated. The palate is long, and 

 the anterior border of the posterior nares is between 

 m- and m". 



Incisors. — The anterior incisor (i^) is very large 

 (ap. 22 mm., tr. 20), and closely appressed in the 

 median line to its fellow of the opposite side; its large, 

 blunt tip lies near the median line; back of this is a 

 wide, oval basin, or pit, bounded by the very heavy 

 posterior cingulum and by the external ridge; the 

 front face is vertically deep (26 mm.). The median 

 incisor (i^) has a low median tip and wide posterior 

 basin. It remotely resembles that of DolicTiorhinus but 

 is far larger (ap. 25 mm., tr. 26) even than that of T. 

 ultimum. The very large canine (ap. 27 mm., tr. 27), 

 as already observed, is long and piercing, with a verti- 

 cal crown length of 57 millimeters, as compared with 

 42 in the paratype of T. ultimum. Its transverse 

 diameter is 27 millimeters, as compared with 22 in T. 

 ultimum. It has similar antero-internal and postero- 

 external edges but is distinguished by its heavier poste- 

 rior basal cingulum. 



Premolars. — The premolars are larger and wider 

 than in T. ultimum. There is little if any postcanine 

 diastema, p' being crowded in behind the base of the 

 canine. Its crown is not preserved, but it appears 

 probable that this was broader — that is, more ad- 

 vanced — than in T. ultimum. In p^, p^, p* the trito- 

 cones are nearly equal to the protocones, and both 



