42 THE NAUTILUS. 



which is a lasidium. In Spatha there are no, or hardly any, hinge 

 teeth, and they are surely not taxodont. The embryo is unknown to 

 me. Yet the above characters of the soft parts fully justify the 

 separation of this genus from the North American Unionidce, and I 

 do not hesitate to affirm, that Spatha should stand in a different 

 family, which may be called Mutelidce, if the genus Mutela should 

 prove to be allied in the structure of the soft parts. Whether the 

 other genera placed by Simpson in this association actually belong 

 here, remains to be investigated. 



DESCRIPTION OF A NEW SPECIES OF TEUNCILLA. 



BY BRYANT WALKER. 



Truncilla lewisii n. sp. PI. Ill, figs. 3, 4, 5. 



Male shell quadrate, subcom pressed; thick, solid; dark reddish- 

 yellow, with faint, radiating lines of green; beaks laterally com- 

 pressed, eroded, but apparently only slightly elevated above the 

 hinge-line, sculpture not seen; anterior end regularly rounded, form- 

 ing an obtuse angle at its junction with the basal emargination, which 

 is nearly straight; dorsal line curved; posterior end slightly emar- 

 ginate and terminating in a broad biangulation, which projects' 

 slightly beyond the posterior and basal lines; a broad, flat groove 

 extends from the beaks to the basal emargination, widening and 

 deepening as it approaches the base; posterior ridge prominent, 

 rounded towards the beak, but becoming flattened and obsoletely 

 biangulated as it approaches the posterior end; immediately in front 

 of the median groove, there is a strong anterior ridge, which be- 

 comes more pronounced as it approaches the base, where it terminates 

 in the angle at the anterior end of the basal emargination, it is more 

 or less roughened by the accentuation of the lines of growth, which 

 elsewhere on the disk are not very strongly developed; dorsal slope 

 concave behind the posterior ridge; interdentum rather long, narrow, 

 rounded and parallel with the hinge; pseudo-cardinals in the left 

 valve, two, the anterior very narrow, straight, directed obliquely 

 forwards and slightly widening towards the anterior end, the poste- 

 rior triangular, the space between them triangular and extending to 

 the hinge; in the right valve, two, the anterior smaller, but well 

 developed, the posterior long, triangular, the space between them 



