174 



testh denoted by an impressed line; cavity of tlie umbo slight ^ 

 but with a deep, obvious and large muscular impression immedi- 

 ately under the beak ; more or less tinged with purple. 



Ohs. The anterior emarginations, above mentioned, are obvious 

 in many species, but are more particularly remarkable in the 

 present shell ; they mark the situations of the tubes of the animal. 



The form and arrangement of the teeth are very much like 

 those of the monodonta, nob., but the outline and other characters 

 of this shell, as expressed in the above description and exhibited 

 in the plate, are widely different. 



It is certainly an inosculating species between Unio and Ano- 

 donta, but traces of the primary and lamelliform teeth are, I be- 

 lieve, always discoverable. It is an inhabitant of the Wabash 

 river, and is not uncommon. 



The largest individual that has occurred, is in length one inch 

 and three-tenths, and in breadth three inches and two-fifths ; but 

 the plate exhibits the usual magnitude. The cavity of the beaks 

 is often more or less tinted with purple. PI. 24. 



SiGARETUS. — Shell ear shaped, univalve, spiral, depressed; 

 aperture very large, entire, embracing a portion of the preceding 

 volution, much wider than long ; destitute of nacre : columella 

 short, spiral : labrum simple : volutions two or three : muscular 

 impressions two, distant : operculum none : a revolving slightly 

 elevated line on the inner surface : spire lateral : periostraca none. 



Ohs. These are marine shells, inhabiting various parts of the 

 globe. Like Ovtda, Oliva, &c., the shell is included within the 

 mantle of the animal. Of the few species yet discovered, Linne 

 considered one a Helix. Lamarck adopted the name of the genus 

 from Adanson, and placed it in his family of Macrostomes, with 

 SlomateUa , Stomafia and Haliotis, distinguishing it from the 

 others by being destitute of nacre. He says, it seems to have 

 j^oine relation to Natica ; and indeed some similarity is exhibited 

 by the S. concavus of that author, and an allied fossil species 

 which Mr. Hoeninghaus sent me under the name of caiialiculatua. 

 But as those shells are external, or not enveloped in the mantle 

 of the animal, the relation must be considered as of analogy rather 

 than of afiinity. 



Cuvicr, who examined the structure of the animal of a specio 

 of this genus, but we are not informed which, either by name ot 



