Mr. Barnes on the Genera Unio and Alasmodonta. 111 — 
sus may be as long as it is broad, while the Nasutus is only 
1 inch, or 10 lines long. The former may weigh more than 
half a pound, the latter less than half an ounce. The for- 
mer may be half an inch thick, the latter, as thin as paper. 
And to say that one is broad and the other narrow, does not 
obviate the difficulty; for these terms are altogether com- 
parative, and, without something for a standard, convey no 
definite ideas. . 
We have therefore adopted an improvement which we 
hope to see become general in the description of Bivalves, 
that is, to give the length from the summit to the opposite 
margin; the breadth between the lateral extremities, and the 
diameter through the disks, at right angles to both the length 
and the breadth; that is, the thickness through the most 
prominent part of the body of the animal. We prefer the 
term diameter to thickness, because the latter is often applied 
to the substance of the shell; the former never. In deter- 
mining these dimensions with ease and accuracy, we have 
constructed a convenient instrument of the following de- 
scription; — 
a, a, is a box-wood ruler, one foot Jong, graduated on its up- 
per side, in inches and lines. 6, c, cross bars, made to stand 
at right angles, and drop down by hinge joints, d, d, upon 
the ruler, for the convenience of packing. The bar, ¢, d, 
slides upon the ruler by means ofa clasp. The shells, to be 
measured, are placed between the bars, and the length is 
read off from below. The instrument measures any irreg- 
ular body or figure, from one line to one foot in diameter. 
When used for measuring shells, it may be called a Con- 
chometer. 
One advantage of thus measuring shells, is, that those of 
the same species, or the same variety, will be found to have 
very nearly the same 2 Sig which will hold good as it 
regards all the varieties of age. ‘These proportions may be 
