OF THE FAMILY OF NAIADES. 413 



the young specimens are sometimes rayed over the umbones. 

 The young aesopus is bright yellow and highly polished. The 

 young verrucosus has sometimes one or two broad interrupted 

 green lines over the middle of the umbones. In young or 

 very perfect specimens of the ellipsis may be seen numerous 

 small rays passing over the umbones towards the posterior 

 margin. In the younger specimens of the rubiginosus indis- 

 tinct rays are sometimes seen. The lanceolatus is yellowish 

 passing into olive. 



It should ever be borne in mind, notwithstanding what 

 has been said above, that colour is exceedingly deceptive, and 

 may often lead to error. It is impossible to find permanent 

 characteristics in it, on which we can universally depend, as 

 locality, exposure, youth, and age so materially affect its ap- 

 pearance. We must therefore consider it in most cases as 

 only auxiliary, though in a few cases it is permanent. 



Beaks. Lamarck, in his generic description of the Unio, 

 says, "natibus decorticatis, suberosis." This character is not 

 permanent by any means in our species, some of which are 

 almost universally found free from decortication, while others 

 are partially so ; and others again rarely free from it. The 

 objection to receiving it as a permanent character even in spe- 

 cies is, that more or less exposure to the action of the stream, 

 &c. will cause the beaks to be more or less eroded in the spe- 

 cies where erosion takes place. Some species, however, seem 

 to resist this erosion with great success, owing, as I appre- 

 hend, to the peculiar firmness of the texture of their epidermis, 

 which certainly differs in different species. I have never 

 seen either of the following species eroded, viz. U. parvus. U. 

 calceohis, U. lacrymosus, U. rubiginosus, or the Symphynota 

 Isevissima (the two last herein described). It is rarely we see 

 a ponderous shell free from this erosion, and the U. cylindri- 

 cus seems to be peculiarly subject to it, for the form of the 

 beaks can rarely be even traced, yet the largest specimen in 

 my cabinet, nearly five inches broad, possesses the epidermis 

 untouched on this part. The beaks of many of the species, 

 when found in a perfect state, are crowned with concentric 



