42 AMERICAN JOURNAL 



variable species, and cite the nearly uniform size of the shells, 

 their similar ornamentation and restricted habitat as proofs of 

 the correctness of their opinion ; there appears to us to be a 

 well founded division of the species into two groups, the one 

 containing shells which are smooth or obscurely tuberculate, 

 and the second those developing distinct spines. 



Endeavors have been made to connect lo fluvialis and spi- 

 nosa the respective types of the two groups, by series of speci- 

 mens, but no fluvialis has been found with better developed 

 protuberances than the shell described by Mr. Reeve as verru- 

 cosa, which is still a long way from the spinosa. In the young 

 shells the differences are very much better shown than in ma- 

 ture individuals, and no one would think of connecting the 

 quite young of the two species. There are very many groups 

 in the other genera of Strepomatidse, in which the species 

 resemble one another quite as closely as in lo; we may in- 

 stance the close resemblance of Angitrema armigera, Dut- 

 toniana and fasciolata ; of verrucosa Raf , {mqjera, Say,) and 

 lima; of geniculata, salebrosa, and subglohos'i; of Ancnlosa 

 prserosa and tseniata ; of the species of ScJdzostoma ; of the 

 heavy cylindrical Goniobases of North Alabama ; and many 

 like instances will readily occur to those who have studied the 

 family. 



The following remarks on the species are illustrated by 

 figures in most cases drawn from the original type specimens :* 



SMOOTH SPECIES. 

 1. lo FLUVIALIS, Say. 



Plate 3. Fig. 1, typical ; fig. 2, variety ; fig 3. verrucosa, Reeve. 



Our figures exhibit the extreme range of variation in form 

 and ornamentation ; there is considerable variety in coloration, 

 from light yellow, through various shades of light and dark 

 green and brown, to black. Some specimens are beautifully 

 banded. A very dark colored shell, not half grown, was des- 

 cribed by Mr. Lea as a distinct species, with the name of tene- 

 brosa. 



He now agrees with me in considering it a synonym of flu- 

 vialis. In the description he remarks, " It seems to me to be 

 very distinct in color. The channel is more curved to the left 

 and backward, than in Mr. Say's species." 



This distortion of the channel is of very frequent occurrence 

 in the genus, sometimes giving it a spiral curve as in lo sjnros- 

 toma, Anthony, and sometimes, when twisted higher up, caus- 

 ing a hump to revolve on the body whorl, as in lo gibhosa, 

 Anthony. 



*I am indebted to the liberality of Prof. Jos, Henry, of the Smithsonian 

 nstitution, for the opportunity of using these engravings. 



