290 DISCOVERY REPORTS 



zygomatic width 140-7 mm., being 55-2 per cent of the total skull length ; and hamulo- 

 premaxillar length 173-2 mm., 67-9 per cent. It is now no longer possible to determine 

 age by the skulls. In these skulls the cranial sutures disappear and the muscular crests 

 usually become emphasized, while in the oldest, or at any rate the most completely 

 ossified specimens, the jugal fuses to the maxilla as it does in the adult male. The 

 palate deepens very much and the pterygoids lengthen until they project far below the 

 palate and eventually the pterygoids fuse with the palatine bones (Plates XI, XII, 

 XIII, figs. 6). 



There are four skulls which are better ossified than any of the others ; the cranium 

 is completely fused, the jugal is united to the maxilla and the development of the ridges 

 and rugosities for the attachment of muscles, considered with the other characters, 

 shows that these are skulls of aged animals. The lengths are 255, 264, 254 and 267 mm., 

 the order being that of body length. It will be noticed that the first and third hardly 

 depart from the average for the series, but the fourth is not only the largest skull but 

 is from one of the two largest animals. All these animals were pregnant when killed. 

 Since these crania are incapable of further growth it is clear that they have attained 

 their full size. 



The rate of growth of the skull, like that of the body, is highest during the first six 

 months in both sexes, when it is 32 per cent for males and about 26 per cent for females. 

 The rate of growth for male skulls falls to 16 per cent between the first and second 

 years and 5 per cent between the second and third years, rising again in the short series 

 of measurements available to 13 per cent between the third and fourth years, and finally 

 to 14 per cent between the fourth and fifth years: thereafter it dechnes to 11-5 per cent. 

 The rate of growth of the female skull falls to 7-8 per cent between the first and second 

 years and continues to decline until it reaches 2-8 per cent between the fourth and fifth 

 years, showing an abrupt rise after that to 10-3 per cent between the fifth and sixth 

 years (Table II). 



It will be observed that, in the female, between the fourth and fifth year, there is a 

 fall in the rates of growth, and that this is succeeded by a rise between the fifth and the 

 succeeding stage. It is possible that this indicates the onset of sexual activity which 

 begins towards the end of the fourth year in the female. Otherwise it is difficult to offer 

 an explanation, unless the apparent fall is the result of accident resulting from the 

 limited number of specimens available for measurement. It may, however, be remarked 

 that the fall appears in the rates of growth of both body and skull. 



DENTITION 

 Of the adult 

 According to Reynolds (1913) the dentition of Otaria is - - = 34 or 36. In 



O. byronia the normal state is - - - - = 36, but the last upper cheek tooth is not 



