THE NAUTILUS. 63 



name which we now adopt for this magnificent Naiad, Unio 

 giganteus Barnes 1823. 



This very appropriate name has been totally overlooked by 

 systematists. Simpson, it is true, mentions it ; but he credited 

 it to Lea, saying, ' ' Dr. Lea does not describe this but only states 

 that it is in Dr. Mitchell' s collection. Probably it has never been 

 described." (Synopsis, 1900, page 767, note 4.) 



Unio giganteus was, however, described as a named variety 

 of his crassus which, as is well known, embraced the plicate 

 shells. (His Variety "jfif," is typical Lake Erie plicata /) 



Following the general description of crassus, Barnes writes 

 "Variety (i) Unio giganteus. Mississippi. Dr. MitcheW s col- 

 lection." The variety (i) deserves particular notice. A single 

 valve sent by Professor Douglass to Dr. Mitchell weighs fifteen 

 ounces. It is in every respect a gigantic shell. The distance 

 between the points of the two lobes of the cardinal tooth is one 

 inch ; the length of the lateral tooth, three inches ; diameter of 

 the posterior cicatrix, one inch, and its depth, one-fourth of an 

 inch. 



This species, of which four specimens were obtained by the 

 N. W. Expedition, might perhaps constitute a separate species 

 under the designation of Unio giganteus. It is three times the 

 size of the largest Unio crassus mentioned by Mr. Say and M. 

 Lamarck. 



Three specimens 



Diameter 2.9, Length 4.8, Breadth 7.2, 



3.0, " 4.6, 7.0, 



3.1, 4.7, 7.1, are pre- 

 served in Dr. MitcheW s collection. 



Another specimen 



Diameter 2.9, Length 4.9, Breadth 7.0, and weighing fourteen 

 ounces, is preserved in Gov. Cass's collection, Detroit. 



Habitat. The Mississippi, near Prairie du Chien. Prof. 

 Douglass. 



These specimens labeled "giganteus," in Dr. Mitchell's col- 

 lection, were mentioned by Mr. Lea, who (in Obs., vol I, page 

 31, footnote) writes "The giganteus of Dr. Mitchell's collec- 



