BREEDING HABITS OF THE OPOSSUM HARTMAN. 



359 



It must not be supposed, however, that all of the eggs develop. 

 Indeed, about 10 per cent of the eggs found in the first half of 

 gestation are defective, the majority of which are actually unferti- 

 lized. Such moribund specimens may be seen among the normal 

 eggs in figures 31 and 36, plates 7 and 8. 



In spite of the progressive mortality of eggs and embryos in utero 

 it happens that very often more young are born than can be accom- 

 modated by the 13 teats in the pouch. The writer has records of 18 

 and 21. The excess is, of course, doomed to death by starvation. 



NUMBER AND ARRANGEMENT OF THE TEATS. 



As just indicated, most female opossums have 13 teats. Of these 

 12 are arranged in the form of a horseshoe with 6 teats on each side, 

 the open side of the horseshoe forward. The odd teat is in the center 

 opposite the third teats, counting from the posterior end (fig. 47, 

 pi. 10). 6-J-1+6 may therefore be used as the formula to designate 

 the usual number found. Just what proportion of animals have this 

 number is not known. Certain it is that many animals vary from 

 this number. The writer made no pretense to gather complete data 

 on this subject; but a perusal of his cage notes yield the following 

 figures : 



NUMBER OP YOUNG TO THE LITTER. 



Although 13 or more teats are usually recognizable in the opossum 

 pouch it is extremely rare to find them all occupied. Still more rare 

 is a number above 13. The writer has never seen more than 13. 

 But in the files of the Biological Survey there are two authentic 

 records of 14 pouch young. One of these was recorded in R. J. 

 Thompson's Field Catalogue and relates to a Virginia opossum found 

 in Tennessee, June 23, 1892. The arrangement of teats is sketched 

 in the notes and is 6+2+6. The other record is found in the E. W. 

 Nelson-E. A. Goldman Field Catalogue of Mexican opossums. The 

 specimen with "14 young in pouch" was taken at Oaxaca, June 25, 

 1895. 



