396 Monograph of the Species of 
selves are minute and difficult to examine ; the num- 
ber of teeth, which is one of the most important 
specific characters, varies at. different stages of ma- 
turity ; and, what is still more unfortunate, Mr. Say, 
who gave names to all the species which have been 
hitherto described, gave us no figures in illustration 
of his descriptions. These are the sources of con- 
fusion, and hence I have received, from our best 
conchologists, a single species under four of the six 
names that Mr. Say applied to his different species. 
It would be well, if, in writing descriptions, the 
teeth were arranged as primary and secondary, in- 
dicating by the primary those teeth which are always 
present when there are any, and by secondary those 
which are superadded in the progress of growth, or 
at maturity. Such a division would be applicable 
in regard to almost every species.. 
I have collected every specimen of Pura which I 
could find in the cabinets of Boston, and have re- 
ceived numbers from Pennsylvania, Maryland, and 
Ohio, and I have carefully examined every individual. 
By means of a stationary microscope of moderate 
power, and by employing a convex lens in such a way 
as to bring the rays of the sun to a focus within the 
aperture of the shell, I have been enabled to ob- 
fain a fair view of the deeply seated teeth. ` 
I have made out some of Mr. Say’s species with 
certainty, and have become satisfied with regard to 
others in my possession, that they were unknown to 
im. Two or three of his species I have not yet 
seen, or have failed to identify them. — 
It is hoped that the accompanying figures may 
