THE NAUTILUS. 19 



and others have placed in this genus, but I believe they have no 

 close relationship to these South American forms, and that they are 

 merely depauperate Unios, which have a close affinity to species of 

 that genus found with them. 



The Uaionichv of Europe, North and Central America, and prob- 

 ably of the whole Northern Hemisphere, develop eggs in the outer 

 gills alone as far as is known, with the exception of Uniomultiplica- 

 tus and one or two others, which contain embryos in all four leaves 

 of the branchiae. Ihering states that in all the Najidae hitherto 

 examined from South America, the eggs are borne in the inner 

 gills. I may remark in passing that the shells of the Australian, 

 New Zealand, and many South African Unios bear an astonish- 

 ing resemblance to those of South America in form, texture, smooth 

 epidermis and concentric, sometimes slightly granulated sculpture, 

 and especially in the peculiarly compressed, parallel cardinal teeth, 

 and Suter states'^ that the embryos of N. menziezl are borne in the 

 inner gills. Ihering calls attention to the fact that all South Ameri- 

 can Unionidce have a radial beak sculpture, and suggests that prob- 

 ably the same character may be found in the New Zealand species. 



I have carefully examined extensive series of Uiiio menziczi and 

 lutulentus, and on the latter find that the umbos are radiately 

 ribbed where the shells are not too badly eroded, and there are 

 traces of such ridges on the former and on some Australian species. 

 He believes that we shall find the beak sculpture one of the best 

 characters for determining the minor divisions of the Unionidce. 

 Notwithstanding the opinion of this eminent conchologist, and the 

 fact that Mr. Wm. A. Marshall, of the New York State Museum, 

 who has also given this subject some very careful study, believes that 

 the beak sculpture is quite constant and may be used in determining 

 species, my own experience in handling great quantities of material 

 from all over the world leads me to consider this a somewhat varia- 

 ble character, and although it will no doubt prove very useful in 

 studying species and the smaller groups, yet I am sure it cannot by 

 any means be always relied on. 



It is only in Europe that the post-embryonic larvse of the Union- 

 idce have been observed actually attached to fishes, though the 

 North Ameiican species are known to possess hooks and bristles 

 during this stage, and they no doubt make use of the same means to 

 assist in their distribution, as do their Old World relations. 



3 N. Z. Jl. of Science, No. 6, Vol. I (new issue), p. 250. 



