FAMILY MYTILID.E — PINNA. 187 



(EXTRA-LIMITAL.) 



Genus Pinna, Linneus. Shell longitudinal, wedge-shaped, equivalve, gaping at the base and pointed at 

 the summit, with the beaks straight and acute: hinge lateral and without teeth ; ligament mar- 

 ginal, linear, very long and half interior. Animal with its foot tongue-shaped, conic, and 

 bearing an ample byssus. 

 P. seminuda. (Lam. An. sans vert. Vol. 3, p. 27.) Shell with the apex very broad, obliquely trun- 

 cated, with longitudinal scaly furrows ; posterior side smooth. Color, reddish grey. Southern Coast. 

 P. muricata. (Id. lb. p. 23.) Shell moderately large, thin, pellucid, subtruncate, with a few muricated 

 longitudinal furrows. Scales small, erect, subacute. Allied to the preceding. Southern Coast. 



FAMILY UNION ID jE. 



Animal with the mantle entirely open beneath, with a particular opening for the vent; beneath 

 this, an incomplete tube for respiration, furnished with tentacular papilla. Foot very large 

 and thick ; without a byssus. Inhabiting fresh water. Shell free, with an epidermis, 

 equivalve, inequilateral, transverse. Hinge variable, sometimes furnished with an irregu- 

 lar simple or divided cardinal tooth, and a longitudinal one, which extends under the 

 corslet ; sometimes irregular granular tubercles in the place of teeth : in some species, 

 entirely wanting. The posterior muscular impression subdivided. 



Obs. This family corresponds with the Naiades of Lamarck, and to a portion of the family 

 Submytilaces of Blainville. It is a well characterized family, which is more than can be said 

 of the genera into which it has been attempted to be subdivided, or many of the species. 

 The form and number of the teeth are so variable, and run into each other by such insensible 

 gradations until they become obsolete, that it has been doubted whether they may not all be 

 reduced to one genus. North America is particularly rich in species. In the latest edition 

 of Lamarck, out of one hundred species, fifty-four* are attributed to the United States ; but 

 this gives but a faint idea of the actual number deseribed by American Conchologists. Say 

 alone has described fifty-eight ; Conrad has enumerated one hundred and sixteen ; and Lea 

 has carried the number beyond two hundred and fifty, most of which have been beautifully 

 figured. There is so much discrepancy of opinion among these writers in relation to the 

 species, and such a variety of forms requiring careful examination, that for fear of r.dding to 

 the confusion, contrary to the plan hitherto pursued, I shall not cite under this family the 

 extra-limital species. 



* Many of these descriptions must have been drawn up from badly characterized specimens j for, in one instance alone, 

 according to Mr. Lea, eight of Lamarck's species are purely nominal, and refer to one and the same species. 



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