AN ICHTHYOSAUSIAN SKULL FEOM QUEENSLAND.— LONGMAN. 251 



dealt with two pairs of occipital elements below the supraoccipital. C. W. Andrews 

 notes that this bone (stapes) " seems to have lost its auditory function, "^o and it 

 is obvious that this so-called stapes cannot be associated with the fenestra ovalis, 

 as Cope supposed. The stapes is usur~lly regarded as the homologue of the hyomandi- 

 bulare of fishes, and a large stapes is recorded for the Cotylosauria. Case figures the 

 stapes of Dimetrodon,^^ a bone which in this and allied Permian reptiles is regarded 

 by Broom^- as the tympanic. The writer is unable to find, however, a parallel in 

 literature to the interpretation of this buttress bone of the Ichthyosaurs as a stapes. 



The columella auris of modern reptiles, the proximal end of which is 

 presumably homologous with the stapes, is always placed antero-laterally to the 

 basioccipital, and is quite distinct in position from this buttress bone. 



The writer suggests that these lower lateral elements in the occipital region, 

 the so-called stapes, should be interpreted as inferior divisions of the exoccipitals. 

 That the upper elements are true exoccipitals seems to be demonstrated by the 

 position of the foramen for the post-auditory nerves, as clearly shown in Andrews's 

 illustrations {loc. cif.), and also by their relations to the foramen magnum. The 

 unusual extension of the intermediate lateral occipitalia, the opisthotics, to the 

 basioccipital, to which they also act as buttress bones, has probably brought about 

 a division of the exoccipitals into upper and lower portions. 



These lov.er lateral elements may thus be interpretated as inferior divisions 

 of the exoccipitals. This change in nomenclature, giving the exoccipitals a ventral 

 extension, appears to be generally supported by the position of the occipital elements 

 in the Permian Tetrapoda studied by von Huene^^ and by R. Broom^*, and in the 

 Stegocephalia illustrated by C. Wiman.^^ It is in consonance with the general 

 arrangement of the bones in modern reptiles, where the exoccipitals are usually 

 the lower lateral elements in juxtaposition with the basioccipital, the opisthotics 

 uniting with them antero-superiorly in adult hfe (distinct in Chelonians) ; these 

 relationships of the two elements are shown by Parker's studies of the development 

 of the skull in the snake and the lizard. ^^ " It is not at variance with Howes and 

 Swinnerton's interpretation of the development of the skull of Sphenodon}" It agrees 

 also 'with the positions given by Kingsley in his diagram of the schematic vertebrate 

 skull. ^'^ Huxley wrote that but for its large size he would have regarded the 

 adjoining bone, now^ generally accepted as the opisthotic, as the stapes. ^^ 



"C. W. Andrews, Marine Kept. Oxford Clay, Brit. Mus., 1910, p. 11. 

 11 E. C. Case, Bull. Amer. Mus. Nat. Hist., XXVIII, 1910, p. 190. • 

 i-R. Broom, Bull. Amer. Mus. Nat. Hist., XXVIII, 1910, p. 223. 

 13 von Huene, Bull. Amer. Mus. Nat. Hist., XXXII, 1913, pp. 315-386. 

 " R. Broom, Bull. Amer. Mus. Nat. -Hist. XXXII, 1913, p. 563, etc. 



15 C. Wiman, Bull. Geol. Inst. Upsala, XIII, 1915, Pt. 1. 



16 W. K. Parker, Phil. Trans. Roy. Soc, Vols. 169 and 170, 1878-79. 

 1' Howes and Swinnerton, Trans. Zool. Soc, XVI, 1901. 



18 Kingsley, Outlines Comp. Anat. Vert., 2nd edit., p. 74. 

 1' Huxley, Anatomy of Vertebratcd Animals, 1871, p. 211. 



