BEAKS OF UNIONID^E l8l 



cases the rays may be seen to begin quite close to the tip 

 of the beak. 



U. ochraceus is easily distinguished from its near relative, 

 U. cariosus, by the much less prominent converging lines of 

 the earlier undulations on the posterior dorsal area and by 

 the lack of converging lines for the later undulations. The 

 undulations of U. cariosus are sharp on the summits and the 

 posterior lateral portion of each undulation is much stouter 

 than any other portion. The undulations of U. ochraceus 

 are round upon the summits and the undulations are not 

 decidedly stouter at one point than at another. Another 

 difference is in the direction of the undulations. If the 

 lateral portions of the undulations of U. cariosus were ex- 

 tended they would intersect the hinge-line posterior to the 

 beaks while the undulations of U. ochraceus if extended 

 would either intersect the hinge-line anterior to the beaks 

 or simply maintain a parallel course with the hinge-line. 

 U. ochraceus presents two entirely different phases of colora- 

 tion, and in this respect the young form agrees with the 

 adult form. 



Not uncommonly, the color is a plain straw-yellow both 

 in the young and in the adult shell. More generally, the 

 ground color is a straw-yellow beautifully ornamented with 

 rays of bright green. In such cases the green rays extend 

 nearly to the tip of the beak, passing over the undulations. 

 This feature, which is common in U. ochraceus is not at all 

 common to other species of Unionidse. A careful examina- 

 tion of numerous specimens of U. cariosus failed to secure 

 an individual which had the adult characters of color dis- 

 tinctly shown in the very young shell. 



Owing to the thin epidermis and fragile nature of the 

 shell the beaks of U. ochraceus are usually much eroded, 

 even in specimens taken from very quiet and very pure 

 waters. 



