1895. PROCEEDINGS OF THE NATIONAL MUSEUM. 327 



It is in tlie cliaracters of the embryo and the shell that we And the 

 great vital distinctions between these families. In the Tiiionida^ the 

 embryo, perhaps without exception, is a (jlochiclimn, whicli is probably 

 characteristic of the nepionic stage of all the genera in the family. 

 The embryo of the South American Mutelid;e has., wherever examined 

 by V. Ihering, proved to be a lasidium, and, although perhaps the 

 relation of the African mutelid groups may not be so close to those 

 of South America as is that of the latter among themselves, yet I 

 have no doubt that their embryos will prove to be something very 

 much like a lasidium. This peculiar stage is, so far as I know, entirely 

 unique among Pelecypods, and though by the character of taxodont 

 teeth the mutelids show afdnities for yucula, Area, Pectunculus, and 

 the like, yet by the evidence of their embryos they seem wholly unre- 

 lated to any other lamellibranchs. 



The irregularly taxodont teeth which characterize the Mutelidai are 

 totally different from the schizodont teeth, which are found more or 

 less developed in every genus of the Unionidic. The peculiar cartilage 

 l)its of Fossuhi resemble to some extent those in Perna, and suggest a 

 possible distant relationship with this and allied genera. On the other 

 hand, it would seem reasonable to suppose that the unionids had their 

 closest afllnity witli other schizodont families. 



II. GEOGEAPHICAL DISTRIBUTION OF THE NAIADES. 



In mapping out the general distribution of the ]S^aiades, although they 

 are all contiued to the fresh waters of the globe, it will be found that 

 they fall into provinces something in the same way as do the other 

 members of the animal kingdom. So nearly do these areas coincide in 

 a number of cases with those of generally recognized regions of animal 

 life, that in several instances I have applied the same names to them. 

 To a considerable extent, as would be expected, these divisions of 

 Naiad life are bounded by the sea, by deserts, and mountain chains 

 which act as watersheds for different river systems. Yet none of these 

 in all cases effectually restrict the distribution of the fresh- water mus- 

 sels; and it is true that in several instances the borders of a xvTaiad 

 region are not marked by any tangible natural barriers. 



The Palearetie Eegion. — This, the largest region of Xaiad life, includes 

 in a general way the whole of Europe, Africa (excepting the Nile), north 

 of the Desert of Sahara ; all of Asia north of the Stanovoi and Altai 

 Mountains, including, probably, the greater part of Afghanistan and 

 Beloochistan, Persia, Arabia, and Asia Minor; and all of North America 

 that is drained into the Pacific. This vast region, covering an area of 

 perhaps 10,000,000 square miles, is inhabited by a single and remark- 

 ably homogeneous Naiad fauna. One si)ecies characteristic of this 

 province, Unio margaritiferus, Linnaius, is found in all parts of Europe 

 except the region along the Mediterranean; also throughout Siberia; 

 in northern Japan, wliich stands on the border between this and the 

 Oriental region, and in that i);irt of this i)rovince in North America 



