EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



513 



nasals, the greater breadth of the malar bridge over 

 the infraorbital foramen, the narrower contact between 

 the postglenoid and post-tympanic processes. 



Against the theory that A . marshi is directly related 

 to B. rohustus is, however, to be noted the important 

 fact that in the premolar teeth the tetartocones are 

 more advanced than in B. rohustus, in which they are 

 singularly retarded in development. The transversely 

 expanded canines constitute another clear distinction. 



Specimens referred to Allops marsM seu crassicornis. — 

 A skull in the British Museum of Natural History 

 collection, London (No. 5743 M), may be regarded 

 provisionally as a very advanced or progressive stage 

 in the evolution of this species, although it exhibits 

 some characters which lead us to regard it as aberrant 

 from the typical A. marshi, especially the somewhat 

 flattened superior section of the horns, which suggests 

 resemblance to Brontotherium leidyi; but the internally 

 placed tetartocones of the premolars differentiate this 

 type from any member of the genus Brontotherium, in 

 which the tetartocones are invari- 

 ably externally placed — that is, to- 

 ward the buccal side of the crown 

 of the teeth rather than toward 

 the lingual side, as in this speci- 

 men. The female sex of this spec- 

 imen is apparently indicated by the 

 small size of the horns and the 

 slenderness of the canines and zy- 

 gomata. The nasals are somewhat 

 long and delicate, cleft distally; the 

 short horns point obliquely out- 

 ward, giving evidence of having been , 

 subflat posteriorly. They are other- ' 

 wise of the general type seen in 

 A. marshi. The zygoma, although 

 crushed, was apparently deep, with 

 slight buccal expansion. The ex- 

 ternal auditory meatus was a large, 

 round opening, as in Menodus and 

 Allops — that is, of mesaticephalic 

 type. The measurements (see table, 

 p. 508) agree better throughout with 

 A. marshi than with any other type, although even 

 in the paratype of A. marshi the tetartocones of 

 the premolars are not so strongly developed as in 

 this specimen. Two well-developed upper incisors 

 are retained on the right side, i' being much smaller 

 than i^. 



The lower jaw, with its flat, deep ramus, pronounced 

 chin, slim, pointed canine (slightly flattened on the 

 internal face), weak, noncrenulate hypoconulid of va^, 

 resembles the primitive type of the Menodontinae in 

 general and to a less degree that of the A. marshi type. 

 Our conclusion is that this animal corresponds more 

 nearly with a very progressive stage of A. marshi than 

 with any other known species. The extremely 

 advanced condition of the tetartocones of the pre- 



molars may, however, entitle it to distinct specific 

 rank. The skeletal parts which are thought to be 

 associated with this skull are described below. 



STcull in the Harvard Museum of Comparative Zool- 

 ogy. — This skull (fig. 431) agrees with the type in 

 the specific characters of the horns and nasals and in 

 the possession of round-topped incisors. 



Progressive characters and ascending mutations in 

 specimens referred to Allops marshi. — The type of 

 Allops marshi (Am. Mus. 501) exhibits a zygomatic 

 index of 67. It differs from the paratype (fig. 429) 

 in the shape of the nasals and in the more retarded 

 premolar tetartocones. There is a trace of the cir- 

 cular pit in the occipital vertex. The tooth row is 

 short (310 mm.). 



Figure 431. — Skull of A Hops marsh 

 Harvard Mus. Front and side views. One-sixtli natural size. 



The paratype, probably a male (Am. Mus. 1445), 

 represents a more progressive stage: (1) it is the broad- 

 est skull referred to this species (zygomatic index 69), 

 the skull being broadly depressed with stout zygo- 

 mata; (2) the tetartocones are very advanced; (3) 

 the occiput is extremely short behind the zygomata; 

 (4) the nasals are thin and do not spread distally; (5) 

 there are traces of a circular pit in the occipital vertex ; 

 (6) the tooth rows are longer than in any other speci- 

 men in the series, namely, 335 millimeters; (7) the 

 same may be said of the molars (average 205 mm.); 

 the grinding teeth are also slightly more elongate; 

 (8) the tetartocones are more advanced than in B. 

 rohustus — in fact, more advanced than in several speci- 

 mens attributed to B. dispar. 



