REPORT ON THE SPHENISCID^. 23 



in accordance with the fact that seven sternal ribs articulate with the breast bone, there 

 are seven distinct depressions on the lateral margin of the sternum. 



In Pygosceles, moreover, the relative lengths of the middle and external xiphoid 

 processes, as compared with thbse of every other species, are reversed. In the latter the 

 external xiphoid processes exceed the middle process in length, while in Pygosceles (PI. VI. 

 fig. 1 2) the central xiphoid process equals or even exceeds that of the external processes. 



The central xiphoid process is relatively narrower in Eudyptes than in Spheniscus. 



Secondly. — In accordance with the form of the sternal keel, the Penguins may be 

 divided into two groups. In the first of these the anterior border of the sternal keel 

 forms nearly a right angle with the plane of the body of the bone, and consequently the 

 anterior-inferior, or furcular angle of the keel projects but little beyond the anterior border 

 of the bone. This form is most strongly pronounced in Pygosceles (PL VI. fig. 5). In 

 the second group the anterior border of the sternal keel forms an obtuse angle with the 

 plane of the body of the sternum, and consequently the furcular angle of the keel is acute, 

 and projects to a greater extent beyond the anterior border of the bone. This form is 

 best seen in Aptenodytes (PL VI. fig. 6). 



Of the other two genera, Spheniscus (PL VI. fig. 4) most closely resembles Ap)teno- 

 dytes in the form of the furcular angle of the carina sterni, while Euclyptes (PL VI. figs. 

 1, 2, 3) occupies an intermediate position between Aptenodytes and Pygosceles, the 

 furcular angle of the keel being less acute than in the former, and more so than in the 

 latter genus. 



In Eudyp>tes clirysocome the anterior margin of the sternal keel is concave, while in 

 both other varieties of the same species it is straight. 



In Pygosceles the keel as a whole is less prominent than in any other genus. 



In the same manner, taking into consideration the size and form of the episternum, 

 the Penguins may be divided into two groups, which coincide with those founded on the 

 form of the sternal keel. 



In Aptenodytes (PL VI. fig. 6) the episternum is entirely absent, while in Pygosceles 

 (PL VI. fig. 5) it forms a broad laterally compressed plate of bone with irregular edges. 

 Both Spheniscus (PL VI. fig. 4) and Eudyptes (PL VI. figs. 2 and 3) present an inter- 

 mediate condition. An episternum is present in every species of both these genera, and 

 presents the form of a slender, laterally compressed spicule of bone, of smaller size than 

 in Pygosceles, but larger than in Aptenodytes. 



Thirdly. — The modifications in the form and direction of the costal processes agree 

 with the peculiarities already pointed out in the configuration of the body and keel of 

 the sternum. 



In Aptenodytes longirostris (PL VI. fig. 10) the costal processes are of large size, and 

 project obliquely forwards and outwards from the anterior margin of the sternum, so that 

 their inner border forms an obtuse angle with the latter. In Pygosceles (PL VI. fig. 9), 



