REPORT ON THE SPHENISCIDJE. 21 



sternal rib, like the vertebral segment to which it belongs (the tenth), is rudimental 

 and easily lost sight of. It articulates by one extremity with the tenth vertebral rib, 

 and by the other it is attached to the preceding sternal rib, about one inch from the 

 margin of the breast bone. Hence, like the seventh sternal segment, it does not reach 

 the sternum. 



I have observed the following variations in respect of the ribs from the arrangement 

 in Eudyptes chrysocome above described. 



In Aptenodytes, Pygosceles, and Eudyptes chrysolophus the first vertebral rib is 

 destitute of the head and neck, and hence in these species, unlike the others, the first 

 vertebral rib articulates only with the transverse process of the first dorsal vertebra, and 

 not at all with the body of that bone. In Pygosceles, moreover, the seventh sternal rib 

 articulates with the lateral margin of the sternum, and in this respect differs from every other 

 species which I have examined, in all of which that rib falls short of the margin of the 

 breast bone. With these exceptions, the arrangement of the ribs is the same in every 

 species. 



According to Reid,' in the Patagonian Penguin there are only seven sternal ribs. It 

 seems probable that the eighth, like the vertebral segment to which it belongs, had been 

 lost in preparing the skeleton. 



The Sternum. 



The sternum of Eudyptes chrysocome from Tristan d'Acunha (PI. VI. figs. 1 and 2), is 

 quadrilateral in form. Its length is twice as great as its breadth. The body of the bone, 

 as distinguished from the keel, is nearly uniformly convex on its lower surface, except 

 towards its lateral margins, where there is a deep longitudinal groove, bounded on the outer 

 side by an oblique ridge, which, commencing at the lateral border of the bone passes 

 obliquely backwards and inwards, and separates the groove in question from the external 

 xiphoid process. This process nearly equals in length the undivided portion of the bone, 

 from the median portion of which it is separated by the posterior sternal notch, and is 

 prolonged backwards in the recent state by a curved cartilaginous bar, the extremity of 

 which nearly touches its fellow of the opposite side. The posterior sternal notches are 

 two in number, and extend forwards from the posterior border of the bone to midway 

 between the latter and the anterior border of the sternum. They are bounded externally 

 by the external xiphoid processes, and in the recent state are filled up by fibrous 

 membrane. The posterior border of the middle xiphoid process, lying between the 

 sternal notches, is somewhat excavated by a slightly-marked V-shaped notch. 



The anterior border of the body of the sternum is prolonged externally into two well- 

 developed "costal processes." These are quadrilateral in form, flattened from above 

 downwards, and project obliquely outwards and forwards. Their lower surfaces are rough 



1 Proc. Zool. Soc, 1835, p. 134. 



