MUSEUM OF COMPARATIVE ZOOLOGY. 59 



spect to the hamuli pterygoidei, the younger skull has these processes 

 longer and stouter than they are in the older. The posterior nares are 

 narrower and higher in the younger, — a difference correlating with the 

 general differences in form of the skull in the two specimens, the nares in 

 the younger being relatively narrow and high as compared with those of 

 the other. The comparative measurements of these skulls already given 

 (p. 49) show definitely the amount of these differences. The palatine 

 surface of the intermaxillaries is less depressed in the older skull. 



In respect to other portions of the skeleton, considerable differences 

 other than those obviously resulting from age are met with. The smaller 

 and younger specimen, which has a girth in the mounted skin (as it 

 doubtless had in life) one fourth greater than the other, has ribs as long 

 as the other. The number of segments in the sternum varies in the two, 

 through the intercalation in the younger specimen of a short cartilaginous 

 one between the eighth and ninth, to which the ninth pair of ribs is at- 

 tached, instead of both the eighth and ninth pairs being attached to the 

 eighth segment, as is usually the case. 



In color, contrary to what would result from age, the younger specimen 

 is much the lighter. 



Asymmetry. — A small amount of asymmetry has now come to be recog- 

 nized as normally occurring in many groups of mammals, from which even 

 the highest are not free. It is most marked, however, in the lower types, 

 and especially in the cetaceans, where it is usually too great to escape the 

 notice of the mo;t cursory observer. The eared seals also exhibit an un- 

 usually great degree of asymmetry. This absence of symmetry doubtless 

 indicates a tendency to a greater than the ordinary degree of individual vari- 

 ation. In the skull of the older specimen of Eumetopias now before me, 

 the asymmetry is very striking, the preponderance of size being on the left 

 side of the skull, which is not only broader, but appreciably longer. Be- 

 sides the asymmetry of size, there is an asymmetry in the position of the 

 different parts, those on one side being in advance of their homologues on 

 the other side.* The following measurements indicate the extent of the 

 asymmetry in size, the measurements being taken from the (homologically) 

 median line outwards at four different points : — 



* This one-sidedness is still more strikingly seen in the above-mentioned female skull 

 of Otariajubnta, especially in regard to the size and position of the postorbital processes. 



Dr. G. A. Maack informs me that in the specimens of the 0. jubeda collected by him 

 on the coast of Buenos Ayres the asymmetry was astonishingly great. On the contrary, 

 he found no asymmetry in the skull of the ArctocejAalus falklandicus. 



