Il2 Mr. Barnes on the Genera Unio and AlasfnodonM. 



called the law of the species, and every Unio which has the 

 same proportions, may be presumed to belong to the same 

 species. 



Another 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 shell 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 his power 

 to settle the question with certainty. 



Writers on Conchology differ very much concerning the 

 right and left, and the base of Biv^alves. 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- 

 ment 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 

 beaks, 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- 

 valve 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 \sfrom 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 3f. Lamarck, there seems to be a proprie- 

 ty in calling the base of a Bivalve, that part which is down- 

 ivard, and from which ihe foot project-; 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- 



