104 THE NAUTILUS. 



subfamily Unionince. The other North American members of the 

 family {Anodontina and Lampsilince) may be disregarded, since they 

 are peculiarly developed and specialized, and have no closer relation- 

 ship with the exotic forms to be discussed here. Of the latter I shall 

 discuss the genera Parreysia and Lamellidens from Asia (India), 

 Spatha from Africa, and Hyria and Tetraplodon from South America, 

 with a few additional remarks on Gastalina^ Diplodon, Glabaris, Fos- 

 sula and Monocondyltza, also from South America. 



THE NORTH AMERICAN FORMS. 



All North American Unionidce, including Margaritana, differ 

 from the African and South American types in two characters, not 

 mentioned by me previously. These are the anterior attachment of the 

 inner gills and the shape of (he palpi. They agree in these with the 

 Asiatic genera. The inner gill decreases perceptibly in width toward 

 its anterior end; the latter is more or less in advance of the anterior 

 end of the outer gill, and it is attached for a longer or shorter dis- 

 tance to the ascending part of the mantle-attachment line, but it is 

 always distinctly separated from the posterior end of the palpi, gener- 

 ally by a considerable interval (see pi. v, figs. 1, 2, 3, 4). Possibly 

 in Unto this interval is shortest; in all other forms (also in the 

 Anodontines and Lampsilintz) it occupies generally the larger portion 

 of the ascending part of the mantle-attachment line. As regards the 

 shape of the pafpi, this character is not so sharply marked, but in the 

 North American forms they are more or less falcate, with a poste 

 riorly drawn out point. The hind margin of the two palpi may be 

 more or less connected or almost free (compare the same figures 

 on pi. VI). 



As to all other characters we are to separate Margaritana from the 

 rest of the North American forms, and I have created for it the sub- 

 family Margaritaninez. As I have pointed out in my first paper, the 

 chief differences are found in the conformation of the anal and 

 branchial openings., in that of the posterior end of the diaphragm, 

 and in the finer structure of the gills. In Margaritana margaritifera 

 (L.), branchial and anal opening form one large, uninterrupted 

 opening, and there is on the mantle margin no distinct separation 

 of them; the branchial is ill defined anteriorly, its papillae passing 

 rather far forward on the inner mantle edge and disappearing gradu- 

 ally, and there is n > supraanal opening. In fact there is no ten- 



