Von Jhering. — On Geographical Distribution of Atax. 253 



animal, and this I find to be somewhat similar to some forms 

 of Unio from the Brazils. The branchiae are fastened through- 

 out their whole length to the mantle and to the abdomen, and 

 it is in the internal branchiae that the eggs and the embryos 

 are contained. Among nine specimens examined by me I have 

 not found any parasites. What is wanted is that observa- 

 tions should be directed to other species, and also that collec- 

 tions of young individuals should be made with apices intact. 

 Nothing is as yet known of the configuration of the beaks 

 (umbones) of the shells of the New Zealand and Australian 

 Unios, nothing at all (as far as I know) of their anatomy and 

 embryology, and nothing about their parasites. 



In my studies on the Unionidce of the Brazils I was struck 

 by the fact that certain parasites on them are somewhat nearly 

 related to European species, while others known to occur in 

 Europe are wanting. For example, I have never met with 

 Aspidogaster in the pericardium, but I have come across several 

 sporocysts, or rediae, in examining the cercaria, especially those 

 of Bucephalus, scarcely different from European species. 



I do not know whether any one has also noticed this in 

 New Zealand or in Australia. I have frequently also observed 

 species of Atax, a genus of (parasitic) Hydrachnids commonly 

 found in the Unios and Anodontas of Europe. The three or 

 four species which I have observed were studied and described 

 by Dr. Koenike, of Bremen, as they were all new. Dr. 

 Koenike has verified the fact recorded by Leidy, that in the 

 Unios of the United States there exists, among other species, 

 one — Atax ypsilophorus — identical with a form found in 

 Europe. With regard to the other species, it will be neces- 

 sary to continue observations in order to learn which are com- 

 mon to the two regions and which are peculiar to each. 



I was therefore curious to see if Atax also occurred in Unio 

 menziesii ; but hitherto I have not met with it. I am not 

 acquainted with the literature and the geographical distribu- 

 tion of the Hydrachnids, especially with regard to those of 

 Australia and New Zealand, and the object of this note is to 

 draw attention to the subject as one which merits attention 

 and study. A complete knowledge of the distribution of 

 animals will enable us in time to reconstruct the ancient 

 geography of the globe in the different geological epochs, 

 so that the presence or absence of certain genera in the 

 different countries or islands [will] enable us to form definite 

 conclusions on the time of appearance of the genus. On this 

 ground the geographical distribution of parasites gives us a 

 method of arriving at their geological age which palasontology 

 does not furnish us with. 



