

76 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



the Orycterope. On the other hand, if the zygomatic arch be naturally incomplete 

 in the Scelidothere, the interspace between the malar and temporal portions must 

 be relatively much less than in the Sloth or Ant-eater ; for the broken end of the 

 temporal part is separated from the obtusely rounded apex of the malar process 

 in the present specimen by an interval of only one inch. 



The articular surface (PL XXIII., fig. 2) beneath the zygoma for the lower 

 jaw is flat and even, with the outer and inner margin slightly bent down, but 

 having no definable anterior or posterior limits ; its breadth is two inches. It 

 differs from the corresponding surface in the Orycterope in being separated by a 

 relatively wider interval from the tympanic bone, and in wanting consequently 

 the support which the bony meatus auditorius gives in the Orycterope to the 

 back part of the mandibular joint. The Armadillos differ still more from the 

 Scelidothere in this important part of the cranial organization, inasmuch as the 

 glenoid cavity is not only protected behind by the descending os tympanicum, 

 but also in front by a corresponding vertical downward extension of the os malse. 

 The Scelidothere in the general form and relative position of the surface for the 

 articulatiorf of the lower jaw resembles the Glossothere more closely than any 

 other Edentate animal with which I have been able to compare it. 



The malar bone of the Megatherium presents, as is well known, two charac- 

 ters, in which it conspicuously differs from that of the Orycterope and Armadillos, 

 and approximates in an equally marked degree to the Sloths ; these characters 

 consist in a process ascending as if to complete the posterior circumference of 

 the orbit, and another process descending outside the lower jaw to give advanta- 

 geous and augmented surface of attachment to the masseteric muscle, in its 

 character of a protractor of the jaw. Now both these modifications of the 

 malar bone are present in the Scelidothere, and are the chief if not the sole marks 

 of the affinity to the Megatherium which the structure of the cranium affords. 

 They are, however, the more interesting, perhaps, on that account, and because 

 they are associated with other and more numerous characters approximating the 

 species in question to the ordinary terrestrial as distinguished from the arboreal 

 Edentata. For if the Scelidothere, instead of the Megathere, had been dis- 

 covered half a century ago, and if its true nature and affinities had been in 

 like manner elucidated by the genius and science of a Cuvier; and supposing 

 on the other hand that the Megatherium instead of the Scelidothere had been one 

 of the novel and interesting fruits of Mr. Darwin's recent exploration of the 

 coast of South America, then the affinities of the Megathere with the Sloths 

 would undoubtedly have been viewed from a truer point than at the time when, 

 — the Scelidothere, and analogous transitional forms, being unknown, — it was 

 regarded as a gigantic Sloth. 



Having indicated the principal characters of the cranium of the Scelidothere, 



