THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



797 



FORAMINA (NERVE, ARTERIAL) OF THE ATLAS VERTE- 

 BRA IN THE TITANOTHERES AND OTHER PERISSO- 

 DACTYLS 



Primitive. — The primitive condition of ttie atlas 

 (fig. 720, A) is as follows: (1) The first spinal nerve 

 issues laterally, the inferior branch passing down 



cit., figs. 28, 29) the groove is bridged over. In the 

 Oligocene genera the groove is open in certain forms 

 (Brontops rohustus; see PL CXCV of this memoir), 

 and bridged in others. This bridging is partly an age 

 character. In all modern tapirs, horses, and rhinoc- 

 eroses the atlantal groove is bridged over. 



-Pattern of an upper molar of five perissodactyls, showing the widely different extent of the 

 enamel edges and surfaces 

 A, Menodus gigaiiteus, an Oligocene titanothere; B, Palaeosyops Tohustus, a middle Eocene titanothere: C, Teleoceras, a Miocene rhinoceros; 

 D, ElasmoilieTium, a very highly specialized Pleistocene rhinoceros; E, Equus, a modern horse. Scale uniform. In A and B the total length 

 of the enamel edges is relatively short; in D and E it is excessively long. The more simple molar crowns, with short enamel edges, are adapted 

 to crushing coarse vegetation; the more complex crowns, with greatly elongated enamel edges, are adapted to the trituration of hard siliceous 

 shrubs and grasses. 



through the "atlantal groove" in front of the pleura- 

 pophysis; (2) the same groove also surrounds a 

 branch of the occipital artery; (3) the pleurapophysis 

 is perforated for a short space on the posterior face 

 and under side by the vertebrarterial canal. 



Secondary. — Progressive characters of the atlas 

 in the Perissodactyla are: 



1. The first character is the bridging over of the 

 atlantal notch or groove by the pleurapophysis, this 

 resulting in a condition like that seen in Tapirus (fig. 

 720, C). This bridging over of the atlantal groove 

 is rare in Eocene titanotheres, although it is repre- 

 sented in a supposed Palaeosyops atlas figured by 

 Earle (1892.1, pi. 13, fig. 29; see bibliography, p. 698); 

 is not present in Oligocene titanotheres. 



2. The loss of the vertebrarterial canal on the lower 

 side of the pleurapophysis. It is a striking peculi- 

 arity that some of the Oligocene titanotheres lose this 

 canal entirely. In some cases this loss seems to be 

 preceded by the constriction of the canal, as in the 

 atlas of Telmatherium ultimum; in others the vertebral 

 artery apparently evaded the canal entirely and passed 

 directly from the axis beneath the pleurapophysis to 

 its possible junction with the occipital artery (fig. 

 720, B). 



Titanotheres. — The titanothere family characters of 

 the atlas vertebra are as follows: 



1. Atlantal foramen open or bridged. The first 

 cervical nerve and a branch of the occipital artery 

 deeply indent or groove the anterior borders of the 

 pleurapophysis; in certain forms (Manteoceras, Lim- 

 nohyops, fPalaeosyops, DolicJiorhinus, see accompany- 

 ing fig. 720) the groove is not bridged over by bone; 

 in other forms {Palaeosyops as figured by Earle, op. 



2. The vertebrarterial canal pierces the inferior 

 lamella of the pleurapophysis in the Eocene Bron- 

 totheriidae as in the modern Tapiridae, whereas in 

 the modern Equidae and Rhinocerotidae it enters 



Figure 719. — Evolution of distinctive generic and 

 specific proportions in the grinding teeth of the Oli- 

 gocene titanotheres 



I, Increasing excess of transverse over anteroposterior diameters and 

 progressive brachycephalic tendency in the Brontops phylum; also 

 illustrating the mean formulae for three species of Brontops. II, Con- 

 trast between the extreme brachycephalic tendency in Brontotherium 

 and Megacerops with the extreme dolichocephalic tendency in Menodus 

 giganteus. In Brontotherium p<-m' are exceptionally broad; in Meno- 

 dus p^-mi are exceptionally narrow. 



the superior face of the lamella. A distinctive pro- 

 gressive feature in certain of the Brontotheriidae, 

 first seen in the upper Uinta forms Telmatherium 

 ultimum and Protitanotherium, is that the verte- 

 brarterial canal tends to close or become obliterated 



