FAMILY MYTILIDAZ — 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 sidesmooth, Color, reddish grey. Southern Coast. 
P. muricata. (to. Ib. 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 UNIONID. 
ANIMAL with the mantle entirely open beneath, with a particular opening for the vent; beneath 
this, an incomplete tube for respiration, furnished with tentacular papille. Foot very large 
and thick ; without a byssus. Inhabiting fresh water, Sue.. free, with an epidermis, 
equivalve, inequilateral, transverse. Hinge variable, sometimes furnished with an trregu- 
lar simple or divided cardinal tooth, and a longitudinal one, which extends under the 
corslet ; sometimes irregular granular tubercles in the place of teeth: tn some spectes, 
entirely wanting. The posterior muscular impression subdivided, 
Ons. This family corresponds with the Naiades of Lamarck, and to a portion of the family 
Submytilacés 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 adding to 
the confusion, contrary to the plan hitherto pursued, I shall not cite under this family the 
extra-limital species. 
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* Many of these descriptions must have been drawn up from badly characterized specimens; 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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