288 ZAGLOSSUS. 



OSTEOLOGY. 



In their original description of the Proechidna Peters and Doria (1876) 

 figured the imperfect cranium on which they based the species. The adult 

 skeleton was next described and well figured by Gervais (1877-78) in the first 

 part of his Osteographie des monotremes. Dubois (1881) has figured the skull 

 and the bones of the limbs, and in a later paper (1884) the elements of the 

 sternum. He also gave brief notes on the skeleton. Thomas (1885, 1887, 

 1907, 1907a) has given cranial measurements of specimens in the British mu- 

 seum, and Weber (18'88) has figured and described other adult skulls. The 

 present discussion is confined to a summary of these notes together with such 

 additional observations as I have been able to make on three other adult skele- 

 tons and the skulls of five younger specimens. 



Skull. — The cranial sutures close early, and by the time that full size 

 is attained, they have become nearly obliterated, although the other bones 

 of the body are still incompletely ossified. Adult crania are therefore practi- 

 cally solid like those of birds, except that the ring-like tympanic bones remain 

 separate from the rest of the skull. The seven crania in the collection of the 

 Museum of Comparative Zoology represent several stages in this progressive 

 ossification from that in which nearly all the sutures are evident, to that in which 

 all have disappeared. The homology of the several bones, owing to the difficulty 

 of f)btaining specimens young enough to show the sutures, is still somewhat 

 unsettled. Sixta (1900) endeavored to homologize the cranial elements of 

 the adult Echidna with those of certain reptiles but van Bemmelen (1900) showed 

 that this was only possible after a study of young skulls. The latter author 

 and Lubosch (1906) have written on the monotreme skull and appear to ha\-e 

 elucidated the main points. Our Proechidna material, however, seems to 

 throw additional light on a few doubtful questions, and presents other details 

 of value. 



The occipital and parietal regions of the brain-case are most developed, 

 a condition which Gregory (1910) notes as primitive among mammals. The 

 exoccipitals are large; the dorsal and lateral margins of each are nearly at 

 right angles where they meet dorsal to the condyles. The large supraoccipital 

 slightly exceeds in breadth the combined width of the exoccipitals, with which 

 it forms the posterior face of the skull. There is considerable variation in the 

 degree to which these three bones share in forming the superior margin of the 



