354 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



sary to go into details here, suffice it to say that it occurs as in other 

 mammals with the exception that during the act both male and fe- 

 male lie on the right side, once the male has attained a hold with 

 his legs about her body and with his teeth upon her neck. 



Both before the female comes into heat and after she passes out of 

 that stage she is extremely combative. The writer has seen a tiny 

 female all but kill a husky male, thrice her weight. 



When a female is in oestrus the courting male makes a peculiar 

 clicking sound with his tongue or teeth. The sound has an almost 

 metallic ring. It is described by Selenka as " ein schmatzender, 

 schnalzender Laut." Females never make the sound and males only 

 at the time of mating. Recently Jones published a paper " On 

 the Habits of Trichosurus vulpecula, the Australian ' Opossum, ' 

 in which he sa} T s : 



During the breeding season, when fights are most common and animals are 

 most vocal, the male produces a curious sound like a sharp licking of the 

 lips and a click of the tongue. So far, I have not heard a female produce this 

 sound. 



It is possible, then, that this clicking sound of the sexually excited 

 male is a point of behavior which is confined to the class Marsupialia, 

 hence may be considered a class character as much so as an anatomi- 

 cal or structural peculiarity confined to the class. 



THE PERIOD OF GESTATION. 



In most mammals ovulation follows immediately or soon after 

 copulation; hence the time between copulation and birth is practi- 

 cally identical with the period of gestation. In marsupials, how- 

 ever, it seems that ovulation is greatly delayed. This refractory 

 period has been designated by Hill as the " postoestrous period," 

 the time between copulation and ovulation. The data so far as col- 

 lected serves to show that the postoestrous period is of considerable 

 length and is a variable quantity. It is largely because of the vary- 

 ing length of this period that the exact length of gestation of no 

 marsupial is definitely known. 



To illustrate the variability of the postoestrous period, Hill's 

 (1913) study of Dasyurus may be cited. This author found 2-celled 

 eggs four, five, and six days post coitum or as the figures may be in- 

 terpreted, ovulation took place two, three and four days post coitum. 



Similar data hold for the opossum. Selenka gives the period at 

 " exactly five times 24 hours " ; but he bases his figures on the finding 

 of eggs in " cleavage, " which were really fragmenting, unfertilized 

 eggs. Between three and four days is nearer the truth. The writer 

 has recovered young opossum vesicles of identical stage from two 

 females (Nos. 298 and 314) 6} and 3£ days respectively post coitum. 



