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Show 62 TilE VOYAGE OF TilE BEAGLE. ZOOLOGY OF ' . b nd capacw. us si.z e of the air-cells which are m therium by the great num er a 1 l er lateral and back parts . . h extend over al t te upp ' ' communication w1th It : t ese b ndary of the foramen magnum : . £: en as the upper ou of the cranial caVIty, as ar. ev . ,l f the basis cranii. The external con-h tenor two-thnos o 1 they also occupy t e an - c Jr. ·l a very inadequate or rat 1er k 11 ld thereJore, auoH . figuration of the s u wou ' b 1 vity were not the existence . f 1 city of the cere ra ca ' . deceptive notiOn o t Je capa "'l . t . ace of the outer and mner f h · ses known L 1e m e1sp and magnitude o t ese smu . h .. s of the olfactory ganglia · arated above t e ongm tables of the cramum are sepc I 'ddle of the cerebrum they are an f th inches· above t 1e ID1 for the extent o ree · . f 1 . . m the interposed air-cells are h lf t at the stdes o t Je ciamu inch and a a apar ; 1 b k t of the cranium about one inch. from one to two inches across; at t Je ac pm The sinuses have generally a rounded :o:~ich in figure 3 a probe is represented Th.e foramen rotundum, o~~t:ua~·e situated close together, within a common as passmg), and the forame~ . The carotid canal (g) opens into the outer transversely oblong depression (z!. . 'd l l which conducts the great fifth side of the commencement of this wt e .c 1f.tn.n~ : . . f s to the outlets of its two chief diVISions. pan To hen eprevter ous b one proJ· ects l·nto the cranial cavity. , in the form ofd a n1 angular rocess with three facets: the foramen auditorium mternum .Ck), an ~ te aque- ~uctus vestibuli are situated on the posterior facet. Immedta~ely ;clund the os Petrosum is the 'foramen lacerum jugulare (Z), situated at t~e .po~nt o. convett·~encde of the vertical groove of the 1a tera1 sm· us, WI' th a groove of similar s1ze co.n mue forwards from above the anterior condyloid canal. 'fhe plane of the mterna1 openm· g of t h'1 s cana1 ( c, fig · 3) is directed obliquely inwards andd backwards, ancl 1 'd 1 the lateral wall of the foramen magnum behind the foramen con y 01 eum s opes outwards to the edge of the condyle. Immediately internal to the for~men condyloideum is a small vascular foramen conducting a branch of the bastlar ~rtery into the condyloid canal, for the nourishment, doubtless, of the great hngual nerve. 1 · l In the relations of the plane of the internal orifice of the anterior condy .Ot( foramen with that of the foramen magnum, we search in vain for a correspondmg structure in any of the Mammiferous orders, save the Edenta.ta: * and among these the Orycterope comes nearest the Glossothere in this respect. In the degree of development of the internal osseous ridge giving attachment to the tentorium cerebelli, the Ant-eaters and Armadillos more resemble the Glossothere than does the Orycterope; in which a continuous bony plate ar~hes .acros~ the cranial cavity : in the Manis a still greater proportion of the tentormm IS ossified, • In the monotrematous Echidna, the large canal for the lingual nerve has a widely different direction and course from that in the placental Edentata. FOSSIL :MAl\ll\IALIA. G3 and it consequently recedes the furthest amongst the Edentata, in this, as in most other particulars of the cranial organization, from the Glossothere. The chief distinctive peculiarity in the cranium of the Glossothere, so far as it can be studied in the present fragment, aml compared with that of other Edentata, is the deep, well-marked, semicircular styloid depression, above described. A question may arise after perusing the preceding evidence, upon which the present fossil is referred to a great Edentate species nearly allied to the Orycter ·opus, whether one or other of the lower jaws, subsequently to be described, and in like manner referable, from their dentition, either to the Orycleropodoid or Dasypodoid families of Edentata, may not have belonged to the same species as does the present mutilated cranium. I can only answer, that those jaws were discovered by Mr. Darwin in a different and very remote Joca1ity,-that no fragments or teeth referable to them were found associated with the present fossil; and that, as it would be, therefore, impossible to determine from the evidence we have now before us, which of the two lower jaws should be associated with Glossotlteriunt ; and as both may with equal if not greater probability belong to a totally distinct genus, it appears to me to be preferable, both in regard to the advancement of our knowledge of these most interesting Edentata of an ancient world, as well as for the convenience of their description, to assign to them, for the present, distinct generic appellations. The figures in Plate XVI. preclude the necessity of a table of admeasurements of the cranial fragment of Glossotlterium. DE 'CRIPTION OP A MUTII,ATED LOWER JAW AND TEETH, ON WIIICII IS FOUNDED A SUBGENUS OF :MEGATIIERIOID EDENTATA, UNDER TilE NA.l\IE OP MY LO D 0 N. THE genus JJfegalonyx, as is well known, owes its name and the discovery of the fossil remains on which it was founded, to the celebrated Jefferson,* formerly President of tile United States. Cuvicr, from an examination of a single tooth, and the casts of certain bones of the extremities, especially the terminal ones, determined the ordinal affinities of this remarkable extinct quadmped.t But while he ~- Transactions of the Philosophical Society of J>hJladclphia, vol. i v. p. 246. t Its relations to the Edentata, prcvionsly conjectured by Dr. \Vistar, aro proved in the Annales du Museum, tom. v. p. 358; its more immediate affinities as an annectant form in that group arc discussed in tho edition of the Ossem. Fossiles, of 1833, tom. v. pt. I. p. 160. |