PHENACODUS  , one of the earliest and most

primitive of the ungulate mammals, typifying the family Phenacodontidae and the sub-order Condylarthra . The typical Phenacodus primaevus, of the Lower or Wasatch Eocene of North America, was a relatively small ungulate, of slight build, with straight limbs each terminating in five complete toes, and walking in the digitigrade fashion of the modern tapir . The middle toe was the largest, and the weight of the body was mainly supported on this and the two adjoining digits, which appear to have been encased in hoofs, thus foreshadowing the tridactyle type common in perissodactyle and certain extinct groups of ungulates . The skull was small, with proportionately minute brain; and the arched back, strong lumbar vertebrae, long and powerful tail, and comparatively feeble fore-quarters all proclaim kinship with the primitive creodont Carnivora (see CREODONTA), from which Phenacodus and its allies, and through them the more typical Ungulata, are probably derived . All the bones of the limbs are separate, and those of the carpus and tarsus do not alternate; that is to say, each one in the upper row is placed immediately above the corresponding one in the row below . The full series of forty-four teeth was developed; and the upper molars were short-crowned, or brachyodont, with six low cones, two internal, two intermediate and two external, so that they were of the typical primitive bunodont structure . In habits the animal was cursorial and herbivorous. or possibly carnivorous . In the Puerco, or Lowest Eocene of North America the place of the above species was taken by Euprotogonia puercensis, an animal only half the size of Phenacodus primaevus, with the terminal joints of the limbs intermediate between hoofs and claws, and the first and fifth toes taking their full share in the support of the weight of the body . These two genera may be regarded as forming the earliest stages in the evolution of the horse, coming below Hyracotherium (see EQUIDAE) . As ancestors of the Artiodactyle section of the Ungulata, we may look to forms more or less closely related to the North American Lower Eocene genera Mioclaenus and Pantolestes, respectively typifying the families Mioclaenidae and Pantolestidae . They were five-toed, bunodont Condylarthra, with a decided approximation to the perissodactyle type in the structure of the feet . A third type of Condylarthra from the North American Lower Eocene is represented by the family Meniscotheriidae, including the genera Meniscotherium and Hyracops .

These, it is suggested, may have been related to the ancestral

Hyracoidea . Teeth and jaws probably referable to the Condylarthra have been obtained in European early Tertiary formations . All Ungulata probably originated from Condylarthra . See H . F . Osborn, Skeleton of Phenacodus primaevus; comparison with Euprotogonia, Bull . Amer . Mus. x . 159 . (R . L.*) PHENANTHRENE, C14HIo, a hydrocarbon isomeric with anthracene, with which it occurs in the fraction of the coal tar distillate boiling between 2700—400° C . It may be separated from the anthracene oil by repeated fractional distillation, followed by fractional crystallization from alcohol (anthracene being the less soluble), and finally purified by oxidizing any residual anthracene with potassium bichromate and sulphuric acid (R .

Anschutz and G .

Schultz, Ann., 1879, 196, p . 35); or the two hydrocarbons may be separated by carbon bisulphide, in which anthracene is insoluble . It is formed when the vapours of toluene, stilbene, dibenzyl, ortho-ditolyl, or coumarone and benzene are passed through a red-hot tube; by distilling morphine with zinc dust; and, with anthracene, by the action of sodium on ortho-brombenzyl bromide (C . L . Jackson and J . F . White, Amer . Chem . Jour., 188o, 2, p . 391) . It crystallizes in colourless plates or needles, which melt at 99° C .

Its solutions in alcohol and

ether have a faint blue fluorescence . When heated to 250° C. with red phosphorus and hydriodic acid it gives a hydride C,4 H24 . It is nitrated by nitric acid and sulphonated by sulphuric acid . With picric acid it forms a sparingly soluble picrate, which melts at 145° C . On the condition of phenanthrene in alcoholic solution see R . Behrend, Zeit. phys . Chem., 1892, 9, p . 405; 10, p . 265 . Chromic acid oxidizes phenanthrene, first to phenanthre..;:-4uinone, and then to diphenic acid, HO2C•C6H4•C6H4•CO2H . Phenanthrene-quinone, [C6H412[CO]2, crystallizes in orange needles which melt at 1980 C . It possesses the characteristic properties of a diketone, forming crystalline derivatives with sodium bisulphite and a dioxime with hydroxylamine .

It is non-volatile in

steam, and is odourless . Sulphurous acid reduces it to the corresponding dihydroxy compound . It combines with ortho-diamines, in the presence of acetic acid, to form phenazines . On the constitution of phenanthrene see CHEMISTRY: § Organic .