170' BULLETIN OF THE NEW YORK STATE MUSEUM 



assume other characters such as spines, as in Unio spinosus, 

 or tubercles, as in U. cornutus. Difference of sex, although 

 it does not affect the appearance of the very young shell, 

 may cause the female adult to assume a form very different 

 from that of the male. 



Due allowance being made for a reduction of size, the 

 form and outline of the young shell are found to agree very 

 closely with the form and outline of the adult, except when 

 the female adult is enlarged to accommodate ova. In those 

 species in which the adult is ornamented with a variety of 

 colors the adult colors as a rule are very different from 

 those of the young. Frequently the beaks, when present 

 in perfection, are stained by some foreign matter in the 

 stream or lake in which the animal lived. In such cases it 

 may be difficult to determine what is the natural color of the 

 beaks. As a rule it ma}^ be said that the color of very 

 young specimens when not affected by foreign matter in the 

 water, is a light ashy, or olive grey, the growing shell grad- 

 ually assuming the colors by which it is known in the adult 

 state. 



On the anterior and posterior dorsal areas of the young 

 shells of many species, there will be noticed several elevated 

 fine lines running from the lateral terminations of the undu- 

 lations towards the beaks. In the text of this paper these 

 lines are called the converging lines of the undulations. 

 In addition to these, there will be noticed several other 

 elevated fine lines radiating from the beaks without a 

 corresponding undulation. If a careful examination of 

 these radiating lines be made it will be seen that they 

 are the converging lines of obsolete or absent undulations. 

 In this paper these lines are called radiating lines. The 

 radiating lines are not continued in the adolescent stage of 

 the shell but terminate abruptly, usually before reaching the 

 point, where in the adult the first line of growth is plainly 

 visible. At the point where adult characters of surface and 

 coloration are assumed, the undulations of the beaks, if con- 

 tinued to that point, abruptly terminate. Thus, externally 



