two New Freshwater Shells. 17 
tooth is very thick, minutely crenated, and is double the length of 
the ligamental plate, which latter is much elevated, broad, and 
terminates abruptly in a sinus extending to the point of the wing : 
adjoining the cardinal teeth are four deep muscular impressions, 
one of which is very large, and two of the others very small; the 
anterior impressions are slight and present nothing peculiar. 
Ob. Two odd valves of this unique shell came into the possese 
sion of Mr. G. Humfrey, A. L.S., many years ago; and were 
sold with part of this gentleman’s collection last spring; the shell 
then passed into the hands of Mr. Mawe; Mr. H. was informed it 
came from the river Oronokoo; this I think a very probable lo- 
ceality, for it has all the characteristics of an American species; its 
massy substance and uncommon size seems, moreover, in unison 
with the force and rapidity of such a vast river. I am not well 
satisfied as to the exact form of the dilated process on the anterior 
side; as in both these valves the edges had been injured and re= 
paired: in the perfect shell this part probably may be more di- 
lated, and may terminate in a form somewhat different from that 
which I have described. 
Warwick, 1824. 
Arr. II. On Indistinctness of Vision, caused by the pre- 
sence of False Light in Optical Instruments ; and on tts 
Remedies, by C. R. Goring, M. D. 
(Communicated by the Author,] 
Opricat instruments in general have within the last century been 
brought to so high a degree of perfection, that it may almost be 
doubted if there remain any real improvement to be made in them ; 
nevertheless, it has appeared to me, that in the humble depart- 
ment of their construction which provides against the admission 
of false light, there is still left some capability of a farther ad- 
vancement towards perfection, which may be effected with advan- 
tage, to the performance of astronomical refracting telescopes, 
Newtonian reflectors, and compound microscopes.—As it is neces = 
sary to understand the nature of an evil before we can cure it, as 
Vou. XVII. C 
