No. 6.] AMERICAN MORPHOLOGICAL SOCIETY. 



335 



The following table shows how the above-mentioned types 

 were distributed among the forty-eight individual opossums 

 examined (twenty-four males and twenty-four females). 



A comparison with the development stages in Echidna 

 aculeata shows, I think beyond the question of a doubt, that 

 the variations in the method of formation of the posterior vena 

 cava in Didclphys, so far as its posterior tributaries are con- 

 cerned, are modifications of a ground plan arrangement similar 

 to that described by Hochstetter for his Echidna embryo 

 No. 45. 



The writer's investigations upon the development of the 

 posterior vena cava are as yet incomplete. So far as they 

 have gone, however (an examination of five 1 5-millimeter 

 embryos), they decidedly favor the above conclusions. 



VI. THE CROSSING OF THE OPTIC NERVES 



IN TELEOSTS. 



G. H. PARKER. 



IN ten species of symmetrical teleosts, in each of which one 

 hundred specimens were examined, the right optic nerve was 

 dorsal at the crossing about as frequently as the left. The two 

 types of crossing (right nerve dorsal and left nerve dorsal) 

 were not correlated with sex and were about equally frequent 



