AMPHIDESMA. 



essential character ; nor why Leach has united three species, 

 namely, the A. B^t/sii^tenue SLud prismaticumy together with 

 Lamarck's Lutraria Listeri under the generic name of 

 Abra : all those three species appear to us to possess the 

 characters peculiar to Amphidesma^ we think therefore that 

 Lamarck has rightly placed them in that Genus. The 

 Tellina flexuosa of Maton and Rackett, Thyatira flexuosa, 

 Leach, should not however remain in Amphidesma, for it 

 has not the internal ligament. 



We believe the shells of that Genus are always trans- 

 verse; they are inequilateral, sometimes very slightly so, 

 and in general form they are either nearly orbicular or 

 oval. (According to Lamarck the sides are sometimes 

 gaping.) Sometimes there is one, sometimes there are 

 two, small slender teeth in the hinge of each vale; two 

 distinct lateral elongated teeth, placed rather near the 

 hinge in one valve, which, however, are scarcely to be 

 perceived in the other. The ligament, as we have before 

 remarked is double, its external portion is slender, rather 

 short, and the internal, which is in general longer and 

 larger, is attached in each valve to a more or less elongated 

 g^i'oove, which begins immediately within the umbo, and is 

 continued within the anterior lateral tooth. There is a 

 very large sinus in the muscular impression of the mantle, 

 and in the large species, a distinct flexuosity in the anterior 

 margin of both valves, as in Tellina. 



We have given two representations of Amphidesma 

 variegatwn^ Lamarck, from the coast of Brazil, and two 

 views of another species from the same coast, not men- 

 tioned by Lamarck ; it is the Tellina reticulata, Linne, 

 published as English by Montagu, under the name of 

 Tellina proficua; we have called it Amphidesma reticulatumy 

 because we think it right to retain its old specific name ; 

 the Amph. lacteum, Boysii, prismaticum, and tenue, all of 

 which are English species, have been already figured, so 

 that we do not think it necessary to add any representation 

 of them. We do not venture to add the Mactra Listeri to 

 this Genus, through Lamarck, who has arranged it with 

 his Lutrariae, seems to have overlooked the external liga- 

 ment, so conspicuous in that shell. 



The species of this marine Genus do not appear to be 

 very numerous ; we do not recollect to have seen any in a 

 fossil state. 



