MOLLUSCS. 



271 



Fig. I&i.—Onio clavus. 



none. This fact has been used by Dr. Lea to divide the numerous species of Uhio 

 into two groups, those with soldered hinge being called syniphynote, and those with 

 the normal structure asymphynote forms. Although we have a vast number of species 

 of Uhio, there is but little of popular interest to be said concerning them. 



Margaritana (or Alasmodon) is closely similar to Unio, the only differences being 

 in the details of the hinge teeth. M. margarUifera, which occurs both in Europe 

 and in the northern United States, is espe- 

 cially noticeable from the fact that it pro- 

 duces pearls which are frequently valuable, 

 equalling those of the true pearl oyster de- 

 scribed on a preceding page. During the 

 Roman occupation of the British Isles, these 

 pearls were famous, and in more modern times 

 the search has been continued, especially in 

 Scotland, Bavaria, and Bohemia. These pearls 

 not unfrequently have a slight pinkish hue, 

 which is permanent. In the United States the 

 search for these has never been prosecuted 

 with any great vigor, although the writer has seen several fine pearls found in New 

 York State. This branch of the pearl fishery can be conducted with more ease and less 

 danger than that of the true pearl oyster, since the species can be found in brooks and 

 rivers which can be waded, and which are so clear as to render the collection of the 

 mussels an easy task. All the pearls collected have a value, no matter how imperfect, 

 since pearls can be properly polished only by the use of pearl powder, and for this pur- 

 pose inferior pearls are as good as those of the best quality. 



The only other genus which needs mention is Anodonta, in which as the name 

 indicates, the hinge teeth are lacking. The species have a light, thin, smooth shell, 

 and, like the preceding forms, are found both in streams and ponds. Regarding the 



names to be applied to our North Ameri- 

 can Unionidse, the utmost confusion .exists. 

 Many forms were described almost simul- 

 taneously by Rafinesque, Say, Hildreth, 

 Barnes, Lea, and Conrad. The descriptions 

 of the latter author, like all his work, were 

 decidedly poor, and it is said that here, as 

 elsewhere, he described, from accounts fur- 

 nished him by people other than natural- 

 ists, shells which he had never seen. To 

 make confusion worse confounded, Rafines- 

 que, in his later years, when his intellect was 

 clouded, named a set of shells which now 

 figure as his 'types.' Besides this, almost 

 innumerable questions of j>riority exist; the 

 slightest variation of outline has been seized upon to foi-m new species, and in many 

 ways the whole subject has been tortured and twisted so that the person who in the 

 future endeavors to unravel the skein will have a task of no small diificulty. 



In the preceding families of Acephals we have found considerable variation in the 

 free or united edges of the mantle, and in the presence or absence of siphonal tubes. 



Fig. 289. —Unio fiexuosus. 



