432 Transactions. — Miscellaneous. 



historical interest in the coming centuries. Mr. Wallace's 

 supposition that land and fresh- water molluscs, lizards, ka., 

 were distributed over the whole Pacific Ocean by the waves of 

 the sea I think to be more than bold. Under such circum- 

 stances, it is but natural that one looks for a less constrained 

 explanation. It is a fact, curiously enough hardly noticed till 

 now, that those land and fresh-water molluscs which appear 

 only in the Tertiary era show a very limited distribution, or 

 else are restricted to a few regions only ; whilst genera of 

 the Palaeozoic era are cosmopolitan. Thus, Ampitllaria and 

 Anodonia, appearing only towards the end of the Cretaceous 

 period, are found neither in Chile, nor in Australia, New Zea- 

 land, or Polynesia. But the genus Unio, which we know to 

 occur already in the Jura formation, is not wanting in the 

 countries mentioned, and may perhaps also be found on the 

 Fiji Islands. In Chile and New Zealand Unio is the only re- 

 presentative of the family Naiadae, likewise in south-east and 

 west Australia; but in north Australia a representative of 

 the genus Mijcetopus is found, more or less allied forms of 

 which occur also in South America, Africa, and southern 

 Asia, and which evidently reached Austraha from the latter 

 country. 



With still wader distribution than Unio, there are in 

 Polynesia representatives of Limncea, Physa, Planorhis, 

 Ancijlm, AmphipepUa, Pupa, Zonites, Succinea, almost all 

 of these genera being known from the Carboniferous. Too 

 little attention has hitherto been paid to this fact. To give 

 the distribution of the Nephropneusta {Pidmonata stijlom- 

 matophora, aut.) is not possible at present, because the 

 anatomy of the genera is not sufficiently known. A few 

 weeks ago it happened to me that I recognised, by aid of 

 anatomical examination, one of our Hyalines to belong really 

 to the genus Microcystis, a genus which, according to its 

 anatomy — absence of the receptaculum seminis — may be con- 

 sidered as one of the oldest. Yet the oldest known Nephro- 

 pneust is a Zonitid from the Devonian, perhaps actually a 

 Microcystis, which is, at any rate, a nearly-allied gsnus. 

 Perhaps Patida, Streptaxis, Stenogyra, &c., belong also to this 

 group of very ancient and cosmopolitan land-shells, whose 

 distribution in space and time deserves a close study. This 

 certainly can only be done after careful anatomical researches, 

 and our knowledge may therefore progress but slowly. 

 Further, many of these animals may have been introduced. 

 All this is less to be feared with fresh-water shells, whose 

 introduction may be intentional only as far as it is done 

 with the spawn of fishes, &c. ; nevertheless, to a smaller extent. 

 On the other hand, mistakes may be more likely expected in 

 their determination. 



