MOLLUSCS. 



271 



none. This fact has been used by Dr. Lea to divide the numerous species of C^/ito 

 into two groups, those with soldered hinge being called syni2)hynote, and those with 

 the noi-mal structure asymphynote forms. Although we have a vast number of species 

 of Unio, there is but little of jjojiular interest to be said concerning them. 



Margaritaiiu (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 noticeal)le from the fact that it jiro- 

 duces pearls which are frequently valuable, 

 equalling those of the true pearl 03'ster 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, esjiecially in if 

 Scotland, Bavaria, and Bohemia. These |iearl> *' 

 not unfrequently have a slight pinkish hue, -^st,^ „ 



which is permanent. In the United States the , 7 



search for these has never been prosecuted 



with any great vigor, altliough the writer has seen several line ])earls found in New 

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

 danger tlian that of the true pearl oyster, since the si:)ecies can be found in brooks and 

 rivers whicli 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 hnperfect, 

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

 jiose 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 tiie 



names to be applied to our North Ameri- 

 can Unionidte, the utmost confusion exists. 

 Many forms were described almost simul- 

 taneously by Rafinesque, Say, Hildreth, 

 Barnes, Lea, and Conrad. The descriptions 

 '>f the latter author, like all his work, were 

 lecidcdly poor, and it is said that here, as 

 Ise where, he described, from accounts fur- 

 nished him by people other than natural- 

 ists, shells which he had never seen. To 

 make confusion worse confomided, 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 jiriority exist ; the 

 slightest variation of outline has been seized upon to form new species, and in many 

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

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



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

 free or imited edges of the mantle, and in the jiresence or absence of siphonal tubes. 



