MILK DENTITION. 223 



lueasiiriiig 730 mm. from the nose to the end of the tail, the skull 

 having a length of 123 mm., the specimen when killed having 

 been probably a few weeks old. The formula of the milk den- 

 tition found by Malm in this species is given as I. J^^J, C. ^, M. |^|. 

 The third or last lower molar he describes as standing over the 

 fourth of the permanent set, and as having two diverging roots. 

 The first of the two upjier milk molars stands over the third 

 permanent molar, and the second (also double-rooted) over the 

 fourth, these milk molars being probably in reahty the second 

 and third respectively, the first having doubtless already fallen, 

 as had all the incisors except the exterior upper ones, owing to 

 the post-foetal age of the specimens. The formula given by 

 Malm corresponds nearly with that of young skulls of Zalophus, 

 j)resently to be noticed, taken probably from individuals one or 

 two months old. 



In a very young skull of Uumetopias stelleri (No. 4703, Nat. 

 Mus., San Francisco, Cal., Dr. W. O. Ayres, labelled by the 

 collector as "three or four days old"), the milk teeth have all 

 fallen (probably by maceration), but the alveoli of 5,11 but the i 

 middle incisors are still distinct, and indicate the following ; 

 formula: I. ~J, C. j , M. I^^. Thus, of the incisors the pres- 

 ence of only an outer pair is indicated, the middle ones, being 

 rootless and probably imj^lanted only on the gum, would leave 

 no trace of their former presence. The alveoli of the molars ; 

 ^how that, both above and below, the middle one was much 

 smaller than the others. These alveoli are exterior to the per- 

 manent teeth, which do not vertically replace them, that of the 

 first upper milk molar being opposite the space between the 

 first and second permanent molars ; the second opposite the 

 space between the second and third permanent molars, while 

 the third is nearly opposite the fourth tooth of the permanent 

 set. In the lower jaw the alveoli of the milk molars are re- 

 spectively just behind and exterior respectively to the second, 

 third, and fourth i)ermanent teeth. 



In three fcetal skulls of Zaiopkus (No. 6156, Mus. Comp. Zool., 

 Nos. 15660, S , 15661, 9 , Xat. Mus., all from the Santa Barbara 

 Islands), the milk teeth are all still in situ, except the middle 

 incisors, which are replaced by permanent incisors that were . 

 apparently about ready to pierce the gum. As in Uumetopias j 

 and Arctocephalus, the middle molar, both above and below, 

 is much the smallest, and is placed very close to the third, leav- 



