By the kind permission of Dr. Moreno, the cranium No. 1 has been vertically bisected to display the character of the cranial cavity and the nerve-foramina. An instructive plaster-cast of the cavity has thus been made by Mr. C. Barlow, the Formatore of the British Museum.
The olfactory lobes are shown to have been well developed, projecting a little in front of the cerebral hemispheres. These hemispheres are together somewhat longer than broad, slightly broader behind than in front, and a little constricted in the middle. They do not overlap the cerebellum, which is relatively large. The origins of the nerves are very imperfectly shown in the cast; only their exits from the cranial cavity are clear. The most interesting are the optic and trigeminal nerves, which pass out of the cranial cavity at first by a common exit, which is soon subdivided by a bony partition into two canals, the former no less than 0.08 m., the latter 0.045 m. in length. The fourth, seventh, eighth and twelfth nerves are also recognisable on the cast; and one prominence of plaster has filled the foramen lacerum posterius.
Compared with the brains of Mylodon and Scelidotherium, so far as known from casts of the cranial cavity,[46] that of Grypotherium is observed to be more elongated, with less divergent and prominent olfactory lobes, less constricted cerebral hemispheres, and a larger cerebellum. In the form and proportions of the cerebrum and cerebellum, it similarly differs from Megatherium.[47] The cerebral hemispheres of the existing Cholœpus didactylus and Bradypus tridactylus[48] are more tapering forward, and their cerebellum is relatively smaller than in Grypotherium.
Auditory Ossicles.
The auditory ossicles were preserved in the tympanic cavities of both skulls, Nos. 1 and 2, being retained by the dried soft parts. They were detected by Prof. Charles Stewart, who kindly extracted them, with great skill, from both sides of each skull. Comparing these ossicles with the fine collection in the Royal College of Surgeons, they prove to be closely similar to those of all the existing Sloths, but most nearly resembling those of Cholœpus didactylus. The malleus is bent exactly as in the latter species, and is of similar shape. As observed by Prof. Stewart, it is remarkable in articulating with the incus not only by the head, but also by a diminutive lower facet, which is in contact with a small facetted process on the anterior arm of the incus. A feeble indication of the same secondary articulation is also observable in Cholœpus; but it is curiously absent in the second specimen of Grypotherium. The two divergent arms of the incus are equal in length, as usual in the Sloths. The stapes is only very slightly perforated in both specimens; while a small circular disc firmly fixed to the incus represents the orbicular bone in the second skull. The auditory ossicles of Grypotherium, therefore, are very different from those of Myrmecophaga, in which the malleus is less sharply bent, the incus has divergent arms of unequal length, and the stapes exhibits a large perforation.[49]
Vertebræ and Limb-bones.
Nearly all the remains of vertebræ and limb-bones are in the same state of preservation as the portions of skull and mandible already described, with adherent cartilage and traces of muscles and ligaments. With some of the ungual phalanges there are also well-preserved examples of the epidermal sheath. As already remarked by Roth, the edges of one sheath probably belonging to the fourth digit of the manus, are quite sharp, and indicate that if the animal walked on its fore feet it resembled Myrmecophaga in the peculiar twist of the manus.
All the specimens in this series seem to have been accurately determined and sufficiently described by Roth. It is only necessary to emphasise the fact that the two shafts of humerus with abraded, not sharply broken, ends have a much more fossilised appearance than any other specimen in the collection, and are deeply stained throughout by ferruginous matter. The small shaft, No. 22, certainly seems to have belonged to an adult animal, as remarked by Roth, and it was probably much smaller than any individual indicated by the other remains.
Skin and Hair.