50 TIIK NAUTILUS. 



U. decHvis, length 85, height 45, diameter 35 mm. 

 U. tetralasmus, length 133, height 70, diameter 50 mm. 

 Second. The heaks of U. declivis ave more nearly terminal than in 

 U. tetralasmus (both Say and Lea mention this fact in describing 

 the species). 



Third. The color of the nacre of U. declivis is " purplish" (vide 

 Say and Lea), while that of U. tetralasmus is always white, very fre- 

 quently dull, with large blotches of olive-brown. This is an in- 

 variable characteristic of the thousands I have collected. 



Fourth. U. tetralasmus Say, is rounded or bluntly pointed pos- 

 teriorly, with a rounded or obsolete posterior ridge ; while U. declivis 

 when perfect is much more acutely i-ostrate posteriorly, as noted by 

 Mr. Say, with a " subcarinated " posterior ridge. 



Finally, these species inhabit different stations, U. declivis being 

 found in rivers (Say's type and Lea's U. geometvicus both came from 

 Bayou Teche, a navigable stream) while U. tetralasmus invariably 

 lives in the " small streams and ponds of the South," as stated by 

 Conrad. An apparent exception being Lea's JJ. symmetricus which 

 he said came from (Alexandria, La.) the Ked River; but he pro- 

 cured his shell at second liand from Dr. Hale, who no doubt was in 

 error, as he assuredly was in the case of other shells said to have 

 come from the same river. These shells can live in localities where, 

 from three to six months at a time, tliere is absolutely no water ; in 

 fact living shells have been thrown out by the plowshare, and hun- 

 dreds have been seen killed by fire sweeping over the dried-up 

 ponds. (See plate III, middle figure.) Tins ahility to withstand 

 droughts is no doubt a cause far the misuvderstauding of the group. 



Mr. Simpson, in his ''Synopsis of the Naiades," says: "and if 

 there were no connecting links, it would be easy to make half a dozen 

 species out of it." If the species happens to grow in constant waters, 

 they form more or less perfect shells, and are easily seen to be distinct 

 species. But on account of their drought-resisting abilities and tlie 

 preference for small streams in the case of tlie tetralasmus crowd, it 

 may easily be seen that a majority of the adult shells have had to 

 resist droughts and live through a succession of dormant stages. 

 During these dormant periods, the mantle of the animal is partially 

 withdrawn and the deposition of the epidermis and columnar layers 

 ceases, but the inner nacre is still deposited. Tlie mantle is especi- 

 ally withdrawn from the end of the prominent rostrated portion of 



