THE MUSEUM 



45 



of rock and soil among the mountains 

 of that region. In evidence of this 

 may be suggested the differences in 

 si/e and appearance of shells from dif- 

 ferent streams. 



In correlating these differences we 

 shall find two most robust los in the 

 Holston River. The roughest shells, 

 as regards the whole surface, occur in 

 French Broad River. The most slen- 

 der and graceful forms are found in 

 Clinch River. The smallest adult 

 forms occur in Powell's River, where 

 there are also other peculiarities ob- 

 servable that contrast curiously with 

 what is known of lo elsewhere. The 

 tubercular and spinous forms of Pow- 

 ell's River exhibit their characteristics 

 (spines and tubercles^ in a more rudi- 

 mentary form than those found else- 

 where, and on averaging; the speci- 

 mens it will be observed that they ap- 

 pear to have one whorl less, and more 

 spines or tubercles on each whorl, 

 than is observable in the more luxur- 

 iant forms of other localities. How 

 much of all these differences in forms 

 here suggested is ascribable to species 

 is not at this time a subject of inquiry. 

 In color, the shells of French Broad 

 River are remarkable for green tints 

 in the epidermis. In the Holston Riv- 

 er the tints are somewhat ferruginous, 

 but not to the same extent observable 

 in the shells of Powell's River; while 

 in Clinch River the epidermis is often 

 of a bright yellow or orange, varied by 

 livid tints which are partaken of by 

 many other univalve mollusks inhabit- 

 ing that stream. 



The reader may possibly have felt, 

 in reading a portion of this paper, 

 some curiosity to know why lo occurs 

 only in the upper waters of the Ten- 

 nesee system of drainage. In the in- 

 troductory paragraph of this paper it 

 was stated that "all these streams, 

 not e.xcepting the upper portions of 

 the Tennesee River itself, have in a 

 greater or less degree the characters of 

 mountain torrents, which in reality 

 they seem to be on a very grand 

 scale." From what we now know of 



lo we may infer that it cannot exist in 

 placid rivers, and the limits of its dis- 

 tribution south in the Tennesee River 

 depends on the character of the river. 

 At the point where the Tennesee be- 

 gins to be a majestic, placid stream, 

 there lo ceases to extend its domain. 

 This very simple inference is a key to 

 the solution of other problems relating 

 to the geographical distribution of al- 

 lied forms in the same great family of 

 mollusks. \'ery many of the univalve 

 mollusks of the Tennesee drainage 

 abound in swift shoal water, among 

 rocks over which the water flows in 

 broken torrents, and nowhere among 

 still waters. The Tennesee River at 

 Mussel Shoals is very prolific in var- 

 ious forms of mollusk life which de- 

 light in a rapid current; but below that 

 point, in the navigable portions of the 

 stream, very many of these interesting 

 species disappear, because the condi- 

 tions are no longer favorable to their 

 existence. 



Thus far, in the main, only the typ- 

 ical lo has been considered. There is 

 another group of shells very nearly 

 allied to lo, knosv:i by the generic 

 designation Angitraiia. Mr. Reeve 

 regarded these shells as properly be- 

 longing in the genus Io;.but his views 

 do not seem to have met with much 

 favor by writers on American conchol- 

 ogy on this side of the Atlantic. Not- 

 withstanding this evident Difference of 

 opinion, there is much reason for be- 

 lieving that Mr. Reeve's position is a 

 good one, for some of the species of 

 Angitrema are apparently related to 

 Elk River, the Cumberland River, and 

 some of its tributaries, as lo is to the 

 head-waters of the Tennesee River. 

 Indeed, it appears to be true that near 

 the point where lo begins to disap- 

 pear in the Tennesee River in Ala- 

 bama, some of the forms of Angitrema 

 replace it. The question of the rela- 

 tive distribution of lo and Angitrema, 

 taken in connection with obvious re- 

 semblances in the shells detected by 

 Mr. Reeve, seem to favor the sugges- 

 tion that Angitrema is but a minor 



