in 



In my paper "On the Formation tod Structure of Shells," in 

 the ' Philosophical Transactions' for 1838 (reprinted by Dr. John- 

 ston, 'Letter on Conchology,' p. 113), I observe, — 



" In some very rare instances the shells (bivalves) an- also reversed ; 

 hut the net is not easily observed except in the aneqnal-valved lands. 

 There were formerly in* the TankerviUe collection two specimens of 



Lucina Children*, m one of which the right valve was a dextral 

 shell, in opposition to the general structure. These specimens arc 

 now in the British Museum Collection." 



The four specimens of this shell which 1 have under my eye pre- 

 sent the same anomaly as the two specimens of Lneina Children* 

 above referred to, that is to say, two of them have the left valve the 

 flattest and furnished with the large lobe on the front of the antral 

 margin, and in the other two it is the right valve which has this 

 form and development ; and I cannot observe any other peculiarity 

 between the specimens than this indifference between the develop- 

 ment of the sides of the animal. So that, as a Lucina Children*, it 

 is impossible to determine which is the normal form of the species. 

 A somewhat similar indifference as to the direction of the shell is to 

 be observed in some land univalve shells, as Bulinma aureue, where 

 the shell appears to be indifferently dextral and sinistral ; but in the 

 genus Stavelia it appears more extraordinary on account of the great 

 difference of the form of the two valves. 



We have just received from China a large species of Muteladce 

 (PI. XLI. figs. 2, 3), allied to XJnio Grayii of Lea, which I do not 

 name, as Mr. Cuming informs me that Mr. Isaac Lea is describing 

 and figuring it in Philadelphia*, which offers a curious peculiarity. 

 These shells have the hinder extremity twisted up on one side 

 somewhat like Area tortuosa, but not so regularly ; and unlike that 

 species, the flexure is not always in the same direction : some have 

 the bend towards the right, and the others towards the left of the 

 animal. . . 



I may observe, that, as far as I have been able to examine, the BUM 

 seems a matter of indifference, for as many of the specimens arc 

 bent to the one side as the other. 



It is to be observed that in Area tortuosa and A. Bemitorta the 

 hinge-line is always straight, and it is only the basal line which i- 

 bent to one side, the valves being slightly unequal, and in fact the 

 shell is not distorted ; while in the Uynu under consideration the 

 upper edge of the shell is bent as well as the lower one, and the 

 shell is truly altered in form by some external cirenin.-tanee. 



The shells appear as if they had been softened and suddenly 

 twisted on one side. It has been suggested that tbis change in the 

 form may be produced by the position which the shell occupies in 

 the mud or under tbe stones near winch it lives; but it i> to be 

 observed that n*»W* generally live snnk in tbe mud, and not lying on 

 one side, and that, like shells which live in an civet position, they have 



ecpud valves, while those that live lying on then side almost always 

 have unequal ones ; and if tbe form depended on 1 1 » i ~ euvumstanee, 



* Triijiietra lanceolata sen conlorta. Lea. 



