747 



MEGATHEIU1 



MKiiATHKRIID.K. 



748 



Teeth of ItyluJo*, showing the depth of their Implantation. The cavity at 

 Uw base of the tooth is seen at figure a. Two-thirds natural size. Owen. 



of this foramen, which gives passage to the nerve and artery of the 

 lower lip, indicates that this part was of large size; and that the two 

 symphyseal processes, which probably were subservient to the attach- 

 ment of large retractor muscles, denote that the motions of such 

 *> lip were free and extensive. The angle of the jaw is produced 

 backwards, and ends in an obtuse point, slightly bent upwards; a 

 foramen, one-third less than the anterior one, leads from near the 

 commencement of the dental canal to the outer surface of the jaw, a 

 little below and behind the last molar tooth ; and this foramen pre- 

 sent* the same size and relative position on both sides of the jaw. 

 Professor Owen finds no indication of a corresponding foramen, or of 

 tymphyseal processes in the figures or descriptions of the lower jaw 

 of the Mtgatherium, nor in that of the Sloths, Ant-Eaters, Arraa- 

 dilloes, or Manioc*, which, be had examined with a view to this 

 comparison. 



Profesfor Owen further observes that in the .V../'W,'i<i the 

 inferior contour of the lower jaw is peculiarly remarkable, as Cuvier 

 ban observed, for the convex prominence or enlargement which is 

 developed downwards from iU middle part ; but in the Mylodon the 

 corresponding convexity is slight, not exceeding that which may be 

 observed at the corresponding part of the lower jaw of the Ai or the 

 Orytttroptu ; and after entering into further interesting detail-', tin' 

 Professor comes to the conclusion that the lower jaw of the Mylodon 

 is very different from that of the Mryat/icrium : with that o! 

 lonys he bad of course no means of coiu]>:>i in^r it. 



" Among existing Edentata," continues the Professor, " the Mylodon, 

 in the form of the posterior part and angle of the jaw, holds an inter- 

 mediate place between the Ai and the great Armadillo; in the form 

 of the auchvlosed symptiysis of the lower jaw it resembles most 

 closely the I'nau, or Two-Toed Sloth ; but in the peculiar external 

 con figuration of the symphyris, resulting from the mamilloid processes 

 above described, the Mylodon prenenU a character whi.-h has not 

 hitherto been observed in any other species of Jtrnla, cither recent 

 or fosidL" 



Two species, Mylodon Ilarlani, founded on the forail described by 

 1 ir. Marian, and Mylodon Darrinii, on that discovered by Mr. Darwin, 

 are recorded by Professor Owen ; and he gives the following admeasure- 

 ment* of the lower jaw of the latter species : 



Inchc*. Lines. 



Length (as far as complete) . . . . 17 6 

 Extreme width, from the ouUido of ouo rauius 



to that of the other 90 



Depth of each rnmus 49 



I/etigth of alveolar series 48 



From furl molar to broken end of nymphyaia . 



Breadth of *ympbyis 37 



I/onKitudinaf extent of symphysis . ..46 

 < ir. uinferenco of narrowest part of each innius 6 9 



II- fnrtli. r observes that the teeth and bones of Mylodon ftaricinii, 

 above described, exhibit all the appearances and conditions of those 

 of a full-grown animal, and that they present a marked difference of 

 sue as compared with those of MfMon Ilarlani, which must have 

 been a much larger animal, for if the lower jaw of the latter species 

 bears the same proportion to it* teeth as that of Mylodon Darvnnii 



docs it must be about two feet in length. ('Zoology of U. II. S. 

 Beagle. 1 ) [MTLODON.] 



.Wi'i/vfArriiim (Owen), a large extinct Edentate Mammal, allied to 

 Mtgnthtrivm and Orycterojnu. 



'I'h" remains on which this genus is founded include the cranium, 

 which is nearly entire, with the teeth, and part of the os hyoides ; the 

 seven cervical vertebra, eight dorsal and five sacral vertebra, both 

 scapula;, the left humerus, radius and ulna, two carpal bones, and an 

 ungueal phalanx ; the two femora, the proximal extremities of the 

 left tibia and fibula, and the left astragalus. 



These bones were discovered by Mr. Darwin at Puuta Alt* in 

 northern Patagonia, and in the same bed of partly consolidated gravel 

 as that wherein the lower jaws of Toxodon and Mylodon were imbedded. 

 All the parts were discovered in their natural relative position, indi- 

 cating, as Mr. Darwin observes, that the sublittoral formation in which 

 they hod been originally deposited had been but little disturbed. This 

 beach is covered at spring-tides, and many portions of the skeleton 

 were iucrusted with Fltutrte ; small marine shells were lodged within 

 the crevices of the bones. 



Sufficient of the cranium remains to indicate that its general form 

 resembled an elongated slender subcompressed cone, beginning behind 

 by a flattened vertical base which expanded slightly to the zygomatic 

 region, and thence contracted gradually in all its dimensions to the 

 anterior extremity. 



llemaina of skull of Scelidotherium, reduced. Owen. 



" The Cape Ant-Eater (Orycleroptu) " [AAKD-VABK], says Professor 

 Owen, " of all Edtntata, most nearly resembles the present fossil in 

 the form of its cranium, and next in this comparison the great Arma- 

 dillo (Datypus giyat, CUT.) may be cited. [ARMADILLO.] On the 

 supposition therefore that the correspondence with the above existing 

 Edentata observable in the parts of the fossil cranium which do exist, 

 was carried out through those which are defective, the length of skull 

 of the Scclidothere must have been not less than two feet The 

 cranium is singularly small and slender in proportion to the rest of 

 the skeleton, especially the bulky pelvis and femur, of which bones 

 the latter has a length of 17 inches and a breadth of not less than 

 9 inches ; the astragalus again exceeds in bulk that of the largest 

 hippopotamus or rhinoceros ; yet the condition of the epiphyseal 

 extremities of the long bones proves the present fossils to have 

 belonged to an immature animal. Hence, although the Si-din 

 like most other Kdcntals, was of low stature, and, like the Megathe- 

 rium, presented a disproportionate development of the hinder parts, 

 it is probable that, bulk for bulk, it equalled, when olive, the largest 

 existing pachyderms not proboscidean. There is no evidence that it 

 possessed a tessellated osseous coat of mail." 



Professor Owen gives a most minute and interesting description of 

 the various parts of the cranium, for which we refer to his memoir, 

 remarking only that the most interestim; features in the region of the 

 temporal bone consist in the fine condition of the tympanic bones, 

 aud the presence of a semicircular pit, immediately behind the tym- 

 panic bone, for the articulation of the styloid element of the hyoid or 

 tongue bone. 



"In these points," observes the Professor, "we trace a most remark- 

 able correspondence with the Qlossothere, and in the separate tympa- 

 nic bone the same affinity to the (>ri/deropu$ as has been already 

 noticed in the more bulky extinct Kdvntal. This correspondence 

 naturally leads to a speculation as to the probable generic relationship 

 between the Olossothere and Scelidothoro. Now it may first be 

 remarked that the styloid articular depression is relatively much 

 larger nnd much deeper in the Olossothere than in the Scelidothere : 

 in the former its diameter equals, as we have seen, ouo inch ; in the 

 Sculi<lotlu:ru it measures only a third of an inch, the whole cranium 

 being about two-fifths smaller. If we turn next to the anterior con- 

 dyloid foramina, which in the Scelidotherc are double on each side, 

 we obtain from them evidence that the muscular nerve of the tongue 

 could only have been one third the size of that of 'the Qlossothere. 

 These proofs of the superior relative development of the tongue in 

 the Olossothere indicate a difference of habits, and a modification 

 probably of the structure of the locomotive extremities; and when 

 we associate these deviations from the Scelidothere with the known 

 difference in the position of tin. oivi|iit.-il plane, which in the Glosso- 

 there corresponds with that in the M nnnrco^haya aud 7fra</v;it, wo 

 shall be justified in continuing to regard them, until evidence to 

 the contrary be obtained, as belonging to distinct genera." 



