684 MISS B. LINDSAY ON THE AVIAN STERNUM. [June 16, 



1840. 0. Montana, Blyth, Proc. Zool. Soc. 1840. p. 77. 



1840. 0. californianu, Blyth, ibid. 



18.t1. O. wiow^awa, Audubon and Bachman (fig.). 



1854. O. tnontana, Richardson, Voyage of H.M.S. 'Herald' 

 (osteological fig.). 



18.57. 0. montana, Baird, Mammals N. America, Survey Reports, 

 p. 673 (fig. of horn). 



1859. O. montana, Schott, U. S. Mexican Boundary Report, 

 part ii. p. 52. 



1871. 0. cawa^fenszs, Blyth (" Zoophilus "), The Field, May 13 



(fig-)- 



1880. O. cervina, Alston, Biologia Centr.-Am., Mammal, p. 111. 



1884. O. montana dalli. Nelson, Proceedings of U. S. National 



Museum, vol. vii. p. 13. 



9. On tlie Avian Sternum. 

 By Beatrice LindsaYj Girton College, Cambridge ^ 



[EeceiTcd June 16, 1885.] 

 (Plates XLII.-XLV.) 



Introduction. 



The most typical and simple form of the sternum is that found 

 in Reptiles, where this bone, although associated with a shoulder- 

 girdle of maximum complexity, and strengthened by the apposition 

 of an interclavicle in the median line, is itself undoubtedly of 

 homogeneous origin, that is to say derived solely from the fusion of 

 ribs. In Birds and Mammals the sternum has been supposed by 

 some authorities to be, on the contrary, a composite structure, 

 containing a supplementary median element more or less distantly 

 derived from membrane-bone, and homologous with the free T-shaped 

 interclavicle of Reptiles. Much has been done to increase the 

 plausibility of that theory by a vague use of the terms " interclavicle" 

 and " episternum." These names, when first introduced, expressed 

 nothing but a certain anatomical position of the parts to which they 

 were applied ; but now that the aforenamed reptilian structure is 

 held by nearly all anatomists to be a membrane-bone, the names 

 given to it inevitably tend to suggest a meaning restricted to 

 particular homologies. But, unfortunately, many authors still apply 

 the said names indiscriminately to any anterior median ossification, 

 or paired ossification approaching the median line, without regard 

 to its origin, whether known or unknown ; and thus tliey uninten- 

 tionally create factitious evidence for the above-named theory, by 

 the continual implication of homologies which have never been 

 satisfactorily proved. Instances in point are afforded by certain 



1 Communicated by Dr. H. Gradow, C.M.Z.S. 



