228 SHELLS AND SHELL-FISH. PART I. 



regular turns of the volutions of the spire, and the form 

 of the umbilicus, was most accurately preserved : they 

 were, in short, freshwater carriers absolute counter- 

 parts of their marine brethren, Onustus. As we can 

 find no notice, or even allusion, to such an extraordinary 

 genus of shells in any writer, we have considered it new, 

 and affixed to it the name of Thelidomus. In regard to 

 its affinity, we suspect that it fills the same situation 

 among the Rotellince which Onustus does among the 

 Trochma : this will make it the most aberrant type, and 

 consequently that which comes nearest to the Helicidce, 

 whose form it actually possesses. The annexed figures 

 {fig. 41..) are taken from the only two 

 specimens in our cabinet which we have 

 either seen or heard of ; the line denotes 

 their natural size. Thus, there is ground 

 for supposing that the passage from the 

 marine Trochidce to the terrestrial snails 

 is marked by one or more fluviatile types, 

 just as is the passage on the other side of 

 the HelicidtB, marked by the Limnacina. The accidental 

 discovery, also, of this extraordinary shell, will probably 

 induce naturalists to a more accurate examination of the 

 fossil turbinated univalves than they have received ; for 

 it is clear, that, although Thelidomus opens the path to 

 the Helicidce, there must be several other forms between 

 the two, either extinct or undiscovered. 



CHAP. VIII. 



THE PHYTOPHAGOUS GASTROPODS CONCLUDED. THE HAHOTIDJE, 



OR EAR-SHELLS, AND THE NATICID^l, OR NERITS. 



(213.) THE HALIOTID^E, or ear-shells, follow the 

 Trochidfs, and, like them, in their typical examples, are 

 of a rich pearly and iridescent substance. They have 



