LECTURES ON MOLLUSCA. 115 



North America, and especially of the drainage area of the Mississippi. 

 The American Unios are the most numerous, the most remarkable, 

 and the most beautiful that are found in any portion of the glohe. 

 There is perhaps a special reason for this provision. In no other known 

 portion of the earth is there so large an area covered with soluble lime- 

 stone. The water of the rivers, being saturated with this, would be 

 unfit for many of its uses, were it not for the immense development of 

 this group of heavy shells. The North American Unios may be re- 

 garded as so many water-filters, absorbing the lime from the water, 

 and preserving it from reabsorption by their strong horny skins. The 

 musk-rats also play an important part in this economy, being nature's 

 great Unio-fishers. They bring them up out of the streams, and leave 

 the shells in heaps on the banks. 



The Unios are too easily accessible to most of the readers of this 

 report to need much description. They have the flaps of the mantle 

 entirely separate, (except between the anal and branchial regions,) not 

 united into breathing pipes; but in the breathing region the edges are 

 fringed. The foot is large, thick, and tongue-shaped, enabling the 

 animal to crawl for considerable distances in case of drought. They 

 are often found half buried in sand or mud, leaving the beaks exposed, 

 which thus become worn away by the acids in the water. But some- 

 times they lie on their sides like oysters ; and at others they fix their 

 narrov7 breathing end upwards. In Europe they are rarely found 

 except in rather deep water ; but in America even large and heavy 

 species will be found barely covered by water, and stemming strong 

 currents. To resist these, the shells of Unio have very stout hinge- 

 teeth, with long interlocking side teeth, inside the strong ligament. 

 But the Margaritana group, which abounds most in quieter regions, is 

 destitute of the side teeth ; and the Anodons, which are thin and tooth- 

 less, inhabit the still and comparatively soft waters of the lakes and 

 ponds. The extreme forms of the Unionids are widely removed from 

 each other ; but between each are so many intermediate shapes that 

 their division into genera, however necessary for the easy identification 

 of species, is a matter of great difficulty. Prof. Agassiz has however 

 found that there are differences in the arrangement of the gills and 

 other organs, which are more or less coordinate with those of the shells. 

 It is very desirable therefore that all persons who have access to living 

 specimens should examine and report on them on the spot ; or at any 

 rate preserve a number of each species in alcohol for future investiga- 

 tion. It was in this family that the bisexuality of the Lamellibranchs 

 was first placed beyond dispute. The shapes of the males and females, 

 especially in the U U. perplexus" group, are so very dissimilar that 

 no persons unacquainted with the subject would be disposed to consider 

 them the same species. This is due to the eggs in the female filling 

 the whole extent of the outer gill ; in some instances, as has been com- 

 puted, to the number of six hundred thousand at once. The fossil 

 species present the same generic forms as the recent, and are found as 

 far back as the Wealden rocks. 



Of the Unio group, with distinct lateral teeth, the following forms 

 belong to North America: Eurinea, Lampsilis, Canthyria, Theliderma, 

 Cunicula, Glebula, Uniomerus, Metaptera, and Plectomerus; to South 



