II. On the molar teeth, lower jaw, of Macrauchenia patachonica , Ow

Abstract

The intraneural course of the vertebral arteries is limited, in the class Mammalia , to the Ungulate Series, and is present in very few of these. Of existing species it characterizes the Camelidæ, occurring also, as shown in Palauchenia , in the fossil form of that family; but this rare disposition of the vertebral arteries was likewise met with in a large fossil Ungulate of South America, Macrauchenia , belonging to the Perissodactyle group. The author therefore communicates, as an appendix to his former paper on Palauchenia , a description, with drawings, of the mandibular dentition of Macrauchenia patachonicha , of the natural size, the lower jaw of that fossil animal being still a unique specimen in the British Museum. It displays the entire molar series, with the exception of the first small premolar: the several teeth in place are described in detail and compared with those of other Perissodactyles. The grinding-surface of the true molars presents the bilobed or bicrescentic type, as in Palæotherium and Rhinoceros but Macrauchenia differs from both those genera in the limitation of the assumption of the molar type to the last premolar, the antecedent ones retaining the single-lobed crown. From Palæotherium it further differs in the last molar being bilobed, as in Rhinoceros , not trilobed. In Palauchenia all the premolars have the simpler structure, as in Artiodactyles generally. Macrauchenia resembles Anoplotherium and Dichodon in retaining the typical dentition, i 3-3/3-3, c 1-1/1-1, p 4-4/4-4, m 3-3/3-3 = 44, and in the uninterrupted course of the dental series, not any of the teeth having a crown much higher or longer than the rest. The-paper is illustrated by drawings.