: 112 Mr. Barnes on the Genera Unio and Alasmodon.a. 
called the law of the species, and every Unio which has the 
same proportions, may be presumed to belong to the same 
es. | 
akbaeies obvious advantage of this method will appear in 
the following remark. The Unio which we have designated 
praelongus, is perhaps the Unio purpurata of M. Lamarck, 
all the terms of his description may be applied, and proba- | 
bly with truth, to our shell. But then, he “ believes that 
his sheli came from the great Rivers of Africa.” This 
caused a doubt. Had he stated the very remarkable pro- 
portions of our shell, the identity would have been instantly 
determined. Had he stated the proportions at all, there 
could have been no doubt. We have put it into Avs power 
to settle the question with certainty. dz 
Writers on Conchology differ very much concerning. the 
right and /eft, and the ae of Bivalves. M. Lamarck and 
the authors of the New Edinburgh Encyclopedia consider 
the beaks as the base, and the opposite parts, the upper mar- 
gin: and they give the following direction for right and left. 
If the shell is placed upon its base or hinge, with the liga- 
behind, then the right and left sides of the shell will 
correspond with those of the observer. Burrow on the con- 
trary considers the opposite part to be the base, and the 
Seis: the summit, and says, “If the shell be placed on its 
base, with the area in front, and the valves be then divided, 
the right valve will be opposite the left hand of the examin- 
er, and the left valve opposite the right.” By placing a Bi- 
yalve in the manner directed, it will be perceived that the 
two are directly opposite, the right of one is the left of the 
other. The view which we have hitherto had of these 
parts, and with which Mr. Say agrees, is expressed in the 
following directions: Place the shell upon its base with the 
beaks upward, and the ligament before, (that is from the ob- 
server,) the right and left valves of the shell will correspond 
with the hands of the observer. With due deference to the 
high authority of M. Lamarck, there seems to be a proprie- 
ty in calling the base of a Bivalve, that part which is down- 
ward, and from which the foot projects when the animal is 
in motion. But when the Unio does not, as some authors 
seem to suppose, move on its beaks. The beaks are up- 
wards, and should therefore be called the back rather than 
the base. This makes a simplicity, in the language of Con- 
