25O JOURNAL OF SCIENCE. 



NOTES ON THE GEOGRAPHICAL RELATIONS 



OF OUR LAND AND FRESH- WATER 



MOLLUSCA. 



BY H. SITTER, CHRLSTCHURCH. 



In No. 4 of this Journal, page 151, an article by Dr. Von Jhering 

 appeared, dealing with the geographical distribution of the fresh-water 

 mussels. He says that Dr. Gunther unites the fresh-water fishes of 

 Chile and New Zealand, and that the study of the Najadas confirms 

 this fact, as Unio mutabUis, Lea., from New Zealand and Australia 

 has its nearest ally in Unio auratus of Chile. The mere resemblance 

 of certain mussels from Australasia and Chile could hardly be taken as 

 a conclusive proof of the former existence of a large continent extending 

 between Australasia and South America, but here Dr. von Jhering helps 

 us out of the difficulty by his most important and interesting discovery 

 of the mode of hatching the embryos. The South American Najada: 

 hatch their embryos in the internal branchiae, while those of Europe and 

 North America perform it in the external branchiae. The embryos of 

 both are very different also. 



In his last letter to me Dr. von Jhering expressed his opinion that 

 the Najadce of Australasia would very likely show the same peculiarity 

 in the mode of hatching the embryos as those of South America, and 

 asked me to work together with him in this direction, as well as to 

 ascertain by a large number of measurements of Unio from many parts 

 of New Zealand, whether we have only one or very few species of 

 Unio with many local varieties, or really 6 to 8 distinct species. 



I made a start with the work at the end of July, when Mr. 

 W. W. Smith, of Ashburton, very kindly sent me several hundred 

 specimens of Unio from Albuiy Creek, and a few from Ashburton 

 River. It was rather late in the season, as Professor Hutton says that 

 our Unio breeds in June, but I was fortunate enough to find a good 

 number containing embryos, all of those mussels being of medium size. 

 On opening the mussels carefully I found the small, white and globulai 

 embryos lying, without exception, before the internal branchia 1 , thus 

 proving that these Unios show the same peculiarity in hatching their 

 embryos, as those of South America. It may be objected that I did 

 not find the embryos in the internal branchiae, but only accumulated 

 before them. To this I may reply that the mussels had been lying- 

 alive in a box several days before they reached me, and according to an 

 observation made by Prof. A. Forel, of Morges, Switzerland, the 

 JVajadce expel their embryos when in want of oxygen. This was 

 certainly here the case. Moreover the embryos are widely different 

 from thosn of the Najadce of the Northern Hemisphere. Anyone! who 

 knows the latter ones would not think it possible, in looking at the 

 embryos of our Unio under the microscope, that they really were 

 embryos of Najadw. It is well known that the embryos of the Najadce 

 of the Northern Hemisphere attich themselves to the slimy skin of 



