90 



Myrmecophaga the tympanic bone soon becomes anchylosed with the 

 other parts of the temporal : it is only in Orycteropus, among the 

 existing insectivorous Bruta or Edentata, that it manifests throughout 

 life the fcetal condition of a distinct bony hoop, deficient at the upper 

 part. The os tympanicum of Orycteropus, however, differs from that of 

 the fossil in forming part of the circumference of an ellipse, whose long 

 axis is vertical ; and in sending outwards from its anterior part a convex 

 eminence, which terminates in a point directed downwards and for- 

 wards. 



The internal surface of the present cranial fragment affords a very sa- 

 tisfactory idea of the size and shape of the brain of the extinct species to 

 which it belonged. It is evident that, as in other Bruta, the cerebellum 

 must have been almost entirely exposed behind the cerebrum ; and that 

 the latter was of small relative size, not exceeding that of the Ass ; and 

 chiefly remarkable, as in the Orycterope, Ant-eater and Armadillo, for the 

 great development of the olfactory ganglia. The antero-posterior extent 

 of the cribriform plate, as exposed in this fragment, is three inches, and 

 the complication of the ethmoid olfactory lamellae, which radiate from it 

 into the nasal cavity is equal to that which exists in the smaller Edentata. 

 The nasal cavity is complicated by the great number and capacious size 

 of the air-cells which are in communication with it: these extend over 

 all the upper, lateral and b:ick parts of the cranial cavity, as far even as 

 the upper boundary of the foramen magnum ; they also occupy the 

 anterior two-thirds of the basis cranii. The external configuration of 

 the. skull would therefore afford a very inadequate or rather deceptive 

 notion of the capacity of the cerebral cavity, were not the existence and 

 magnitude of these sinuses known. The interspace of the outer and inner 

 tables of the cranium are separated above the origins of the olfactory 

 ganglia for the extent of three inches : above the middle of the cerebrum 

 they are an inch and a half apart ; at the sides of the cranium the inter- 

 posed air-cells are from one to two inches across ; at the back part of 

 the cranium about one inch. The sinuses have generally a rounded form. 

 In this remarkable structure the present fossil corresponds with the skull 

 of the Mylodon robustus. 



