48 
LUTRARIA ambigua. 
TAB. CCXXVII. 
Spee. Cuar. Transversely elongated, gibbose, 
slightly recurved, anteriorly gaping, with se- 
veral oblique diverging ridges. 
Very variable in the width, and the elevation of the 
ridges : some specimens are nearly obovate, these have 
in general the strongest ribs, which are sometimes 
knotted; other individuals are oblong, with obscure 
costex. 
The longer varieties of this shell approach so nearly 
to the L. gibbosa, (tab. 42.) that the Rev. W. D. 
Conybeare brought me a series, from which I have se- 
- lected those of an intermediate form for figuring, to 
prove their relationship ; and the shortest seem so near 
to Cardita producta, (tab. 197.) that it is very doubtful 
whether they be distinct species. As to the genus we 
are totally in the dark with respect to all the shells of 
this radiated character, not having seen the hinge of one 
of them; and the strong resemblance of the longer 
formed ones to the Lutraria, makes that the preferable 
genus to place all of them in for the present. 
