mOCEE DINGS OF THE NATIONAL MUSEUM. 325 



I bave discussed the genus somewliat at length under the head of 

 Solenaia in this paper, and nothing more need be said regarding the 

 shell. 



The palpi, as in other genera of the Mutelidre, are longer than wide. 

 The mantle is open all around, there being no distinct branchial si[)hon. 

 The anal siphon is only indicated by an oval aperture witli a low border, 

 and it is separated from the branchial opening by a sort of bridge. The 

 branchia" are very large, and the Adams Brothers state that the outer 

 ones are entirely grown together.' 



The foot is an enormous and greatly modified organ, very long and 

 cylindrical, and near the lower part contracted like the neck of a bot- 

 tle. From this the base swells out into a large l)utton, which d'Orbigny, 

 in the magnificent figure in his great work on South American niol- 

 lusks, has represented as covered with low, rounded protuberances. 

 The wall of the burrow corresponds to the shape of the animal, being 

 narrowed in near the button and expanded above and below, and the 

 foot could not be withdrawn unless its lower end was contracted. The 

 unionoid characters of the animal have induced some authors to place 

 it in the UnionidiTe: l)y others it has been considered the type of a sep- 

 arate family, Mycetopodida?.^ But as it is known that other unrelated 

 ISfaiades burrow in the same way, some of which have a strikingly similar 

 foot, and that the shell has a wonderfully soft, silvery nacre, and that 

 it never has a vestige of cardinal or lateral teeth, but sometimes faint 

 traces of taxodont denticles, I think it may be safely placed in the 

 Mutelidffi. The genus is found from southern Brazil northward to 

 Central America. It may be here remarked that all the members of 

 the Mutelid* as herein classified are confined to Africa and South 

 America, with the exception of a few Glabans, which go up as far north 

 as southern Mexico, and a single Central American Myeetopoda. 



From the foregoing characters of the difierent genera placed in the 

 Mutelida^ we may deduce the following family description: 



Shell generally without sculpture or angularities, smooth or rarely 

 slightly sulcate, covered with a tliick epidermis; beaks nearli/ or quite 

 destitnte of scKl2)ti(re, and never e.vltibitinf/ the remains of an embryonic 

 shell; nacre of a peculiarly soft, rich texture, silvery, coppery, lurid or 

 greenish, f/enerally surrounded by a wide, distinct prismatic border; hinge 

 with or without teeth, which, when present, are always irrcyularly taxo- 

 dont, and showhifj vestiges of this Mnd of dentition in occasional specimens 

 in all the genera; escutcheon large, triangular, and distinctly marked; 

 muscular impressions variable; pallial line usually simple, but in some 

 cases more or less inflected into a sinus posteriorly. 



Animal: Labial palpi large, oval or rounded below, and usually ?t'/7/t- 

 out free points, scarcely united posteriorly ; outer gills attached lirnily on 

 each side to the mantle and abdomen, so that the suprabranchial cham- 



'Geneia of Recent Mollusks, II, p. 504. 



^Gray bestowed this iiuiue iu the "Syuopsia of the British Museum" in IS40. 



