1 882.] Recent Literature. 231 



by Professor O. Beccari at Amboina and by this explorer, who 

 went in company with D'Albertis to New Guinea, together with 

 collections from other parts of the Malay Archipelago. Dr. Tho- 

 rell prefaces his work with valuable remarks on the geographical 

 distribution of the spiders of this region and gives a full account 

 of what has been done in the field by his predecessors. 



The Distribution of North American Fresh Water Mol- 

 lusca. 1 — Professor Wetherby's endeavor, in this interesting article, 

 is' to trace the causes which have led to the great differentiation 

 of the fresh-water mollusks and to distinguish the various" faunae. 

 The Limneidae, circumpolar in their distribution, are most abun- 

 dant in the lake region of the Archaean lands, and are essentially 

 lacustrine, although a kw are fluviatile. The Unionidae are most 

 abundant in the region drained by the Ohio, and the typical Ohio 

 forms are continued across the Mississippi to the Rocky Moun- 

 tains and southward to Texas, but in vastly diminished numbers. 

 South of the Ohio and east of the Mississippi, both within and 

 without the Ohio drainage, many of the Unionidae are evi- 

 dently closely related to Ohio types, but along with them, princi- 

 pally in small mountain streams, occur species which have a very 

 different facies, and belong to a different fauna. Such are U. 

 spinosus and U. collinus, the only spinous Uniones. 



The Strepomatidae first appear in New York, and are almost 

 confined to the district occupied by the peculiar Unionidae just 

 mentioned. They do not cross the Mississippi, and are chiefly 



The Unionid genus Anodonta is abundant with the Limneidae 

 of the Archaean lake regions, and plentiful over the northern 

 part of the region occupied by the Uniones, but gives way south- 

 ward to Unio. Most of the described species of Anodonta and 

 Unio are mere varieties, and even Dr. Lea has to confess that he 

 can find no satisfactory anatomical differences in the latter genus, 

 yet there are many types that must be called species. 



Reviewing these facts, Professor Wetherby concludes that the 

 Limneidae form the oldest fauna, and that the typical Ohio forms 

 spread from the Palaeozoic lands of the Northern States, and are 

 older than those found in the Mesozoic and Tertiary regions of 

 the South. 



These latter he refers to a Palaeozoic ancestor whose home was 

 in the western archaean region. 



All fresh-water mollusks were originally lacustrine, adapted 

 themselves first to the change from salt to fresh water, and after- 

 wards to the more rapid change caused by the elevation of moun- 

 tain ranges, and the conversion of lakes into flowing water. 

 Hence the most striking and peculiar forms are found in the 

 mountain streams of newer regions and have not yet had time to 



l Onthe Geographic,?! Distribution of Certain Fresh Miter W/usts of North 

 America. By A. G. Wkthkrby, A. M. Jour, t'mem. S >c. Nat. Hist., July, 1SS1. 



