118 FAMILY CHARACTERISTICS. 



as a test of the accuracy of the views I enter- 

 tained upon this subject, and which perplexed 

 and baffled me for years. It was that of our 

 fresh-water Mussels, the Family of Unios. There 

 is a great variety of outline among them, — some 

 being oblong and very slender, others broad with 

 seemingly scmare outlines, others having a nearly 

 triangular form, while others again are almost 

 circular ; and I could not detect among them all 

 any feature of form that was connected with any 

 essential element of their structure. At last, 

 however, I found this test-character, and since 

 that time I have had no doubt left in my mind 

 that form, determined by structure, is the true 

 criterion of Families. In the Unios it consists 

 of the rounded outline of the anterior end of the 

 body reflected in a more or less open curve of 

 the shell, bending more abruptly along the lower 

 side with an inflection followed by a bulging. 

 This bulging corresponds to the most prominent 

 part of the gills, to which, in a large number of 

 American Species of this Family, the eggs are ex- 

 clusively transferred, giving to this part of the 

 shell a prominence which it has not in any of the 

 European Species. At the posterior end of the 

 body this curve then bends upwards and back- 

 wards again, the outline meeting the side occu- 

 pied by the hinge and ligament, which, wheu 

 very short, may determine a triangular form of 



