328 



NA TURE 



[Feb. 24, 1876 



the maxillaries in front of the orbits. Their molar teeth 

 were of a simple Pakeotheroid type, and the incisors and 

 canines were very much reduced. Their fore feet had 

 four and the hind feet three short stout toes. These 

 animals which, according to the correct rules of nomen- 

 clature, should constitute the family Titattothcrido', be- 

 came extinct, apparently without successors, in the Mio- 

 cene age. 



In the Pliocene period, Rhinoccrotida:, in the form of 

 Aceratherium and Aphclops, were still abundant, though 

 towards the end of the period they became entirely 

 extinct. True rhinoceroses, like those of the Old World 

 with median horns, have never been met with in America. 

 Remains of tapirs are also found but sparingly, and a great 

 development took place in the various forms of three-toed 

 horses, Protohipptis and Hipparion, which replaced the 

 AttchitJieriiwt of the Miocene. These in their turn gave 

 way to true horses, of which remains of several species 

 have been found in Pleistocene deposits, and scattered 

 throughout almost every region of the Continent from 

 north to south. These also became entirely extinct before 

 the discovery of America by the Spaniards, a most 

 remarkable circumstance when the fitness of the country 

 for their maintenance, proved by the facihty with which 

 the descendants of horses introduced by the invaders 

 have multiplied in a feral state, is considered. The tapir, 

 in several modifications, still lingers in some districts of 

 South and Central America, the sole direct representative 

 of the vast and varied perissodactyle fauna of ages long 

 gone by. 



The remains of Artiodactyles in the hitherto explored 

 American Eocenes are very scanty and unsatisfactory as 

 affording indications of their characters. Towards the 

 close of the period only do we find evidences of well-de- 

 fined selenodont {Ag7-iochcerus) and bunodont {Elot/iermm 

 and Platygonus) forms. No species corresponding to 

 the European Anoplotherium, Dtchodon, or Xipodon, have 

 been discovered. During the Miocene period, however, 

 Artiodactyles of both types abounded in North America. 

 The selenodonts were chiefly represented by Oreodon, 

 and the allied earlier and more generalised Ae;riochoerus. 

 These were numerous in species and individuals, but they 

 became nearly extinct by the end of the Miocene, only 

 surviving in the form of Mcrychyus, the most modified 

 form of the group, into the Pliocene. The first indication 

 of the camel-like animals appears in this period in the 

 form of Poebrotherimn J also a few of the generalised 

 Pecora are now met with, allied somewhat to TraguHna, 

 as Leptoineryx, Hypisodus, and Hypcrtraguhis, the latter 

 a very minute species, but no true deer, Bovidce or even 

 Traguli, and no giraffes, Helladotheria or Sivaiheria. 

 The bunodonts were chiefly species of Elothertutn, and 

 an allied four-toed form, Pclonax, remarkable for horn- 

 like lateral processes on its lower jaw, near the symphysis. 

 Peccary-like forms also are now met with. In the Plio- 

 cene or Pleistocene periods, except the somewhat pro- 

 blematical Cosoryx, founded on some branched horn 

 cores or antlers of a form perhaps allied to Antilo- 

 capra from the Niobrara Pliocene, all the animals can be 

 assigned to existing families. Of the Suina, all belong 

 to Dicotyles, or Peccary (which had formerly a more 

 northern range than at present), or to the allied genus 

 Platygotms, no true Stis, or Phacochcerus, Babirtissa, 

 or Hippopotamus^ having been found. Thus the Ameri- 

 can bunodont Artiodactyles, instead of undergoing great 

 and diverse modifications as did the corresponding ani- 

 mals of the Old World, have been gradually dwindling 

 and contracting to the two closely allied species of 

 Peccary, amongst the smallest and most insignificant of 

 all the pigs, and a very old form, having existed (if teeth 

 alone are sufficient evidence) since the Miocene age. 

 The CamelidcB, on the other hand, appear to have 

 flourished, the species being numerous, and the indi- 

 viduals attaining very large dimensions. It is probable, 



in fact, that the family may have originated here, as a 

 tolerably complete series of transitions have been traced 

 from the Miocene Po'cbrotheriiivi., through Procameltis 

 and P/iaitcJienia, to the modern Llamas, which, though 

 now confined to the south, once overran the North Ame- 

 rican Continent. If this view is correct, the Asiatic 

 camels must have come into that country 1)y immigra- 

 tion. A few traces of Ccrvidcs have been found in 

 American Pliocenes, but their paucity, compared with 

 the Old World, until the Pleistocene epoch, would lead to 

 the belief that they cannot have originated there, but 

 must have been imported from other lands. The same 

 applies to the hollow-horned ruminants, of which no 

 forms different from those now existing have been found 



in the fossil state. 



{To be continued.) 



THE EFFECTS OF THE SUN'S ROTATION 

 AND THE MOON'S REVOLUTION ON THE 

 EARTH'S MAGNETISM 



ATtyHEN the mean horizontal force of the earth's mag- 

 ^^ netism for each day of the year has been deduced 

 from well-corrected observations of the bifilar magneto- 

 meter, and the results have been projected in the usual 

 way, the curves thus obtained show successions of maxima 

 and minima occurring in some instances at nearly equal 

 intervals and in others abruptly and apparently without 

 law. It has been found that these changes are experienced 

 similarly at all stations where observatories have been 

 placed on the earth's surface ; they are therefore varia- 

 tions of the magnetic force of the whole earth. The 

 results now considered, though derived from the observa- 

 tions at a single station, may thus be accepted as true 

 generally for all places. 



In the projection of the daily mean forces observ^ed at 

 Makerstoun in 1844, the first and last quarters of the year 

 showed large oscillations of the earth's magnetic force, 

 the maxima occurring near the times of new moon and 

 the minima near those of full moon ; the ranges of the 

 oscillations were not equally great, and the oscillation dis- 

 appeared in the months near midsummer. The mean 

 result for the whole year seemed to show that great 

 changes of the earth's magnetic force were due to the 

 moon's position relatively to the earth and sun ; but no 

 explanation could be offered for the apparent irregulari- 

 ties in the lunar action. Eleven years later (in 1857), 

 while discussing observations made near the equator, 

 I became persuaded that the variations in question were 

 really due to the sun's rotation on his axis. The re- 

 sult of a re- examination of the Makerstoun observations 

 gave a mean period of nearly twenty-six days for the most 

 probable duration of the magnetic oscillation. 



Astronomers who till then had occupied themselves 

 with the determination of the time of the solar rotation 

 had found for it from 27'3 to 277 days. It was difficult, 

 in the face of this result, to suppose that the magnets were 

 better acquainted with the true time of the sun's rotation 

 than the eminent observers, who, with the best telescopes, 

 had watched the movement of the solar spots ; and it 

 was suggested that a movement of the sun's magnetic 

 poles might explain the difference of the periods obtained. 

 More recently, however, it has been found that the spots 

 give considerably different times for the sun's rotation, 

 and especially that those nearest the solar equator indi- 

 cate, as Spoerer has shown, a period of 26*3 days, thus 

 approaching nearly to that obtained previously from the 

 magnetic observations. Dr. Hornstein, director of the 

 Prague Observatory, discovered, independently, nearly 

 the same period from his observations in 1870. 



There still remained for explanation the irregularities 

 already noticed in the lengths and ranges of single 

 oscillations. I, on a reconsideration of all the dis- 

 cussions previously made by him, arrived some time ago 



