\K\V FKESH AVATKll AND LAND SHELLS. 41 



of the two speciiiieii.s now before me, are so precisely the same, that when the odd 

 right valve is placed against the left of the other, they fit perfectly, proving that the 

 curve is a mathematical law belonging to the species. The specimen now figured 

 belongs to II. Cuming, Esq., of London, and is nearly perfect. My single valve was 

 recognized by Mr. Cuming at once, on his recent visit to this country, to be the same 

 as that in his cabinet, and he promptly and liberally oifered to place this unique 

 specimen in my hands, for a more perfect description and figure than could be given 

 of the single immature valve I already had. Mr. Cuming's specimen is nearly six 

 inches along the dorsal line to the angle of the posterior margin, and this line deflects 

 quite two inches in the course of the six inches from a straight line, and it is so 

 twisted that the basal margin at the posterior end turns up to be on a plane with the 

 dorsal line at the beaks. This remarkable curve causes the long lateral teeth to have 

 a curve of double curvature. The greatest distance from the dorsal to the basal 

 margin is one inch and three-tenths. The greatest diameter is nine-tenths of an inch, 

 which gives it a sub-cylindrical form, and it is not unlike a twisted banana, {Mu-sa 

 Hapientum.) This perfect specimen pi'esents the superior portion of the valves covered 

 with small, regular tubercles, which was not suspected in the original, somewhat 

 imperfect single valve, there being on it only a few irregular undulate markings. Mr. 

 Cuming's specimen settles the doubt as to habitat. He received his from Shanghai, 

 and therefore Mr. Fitch and Dr. Ingalls, to whom I owe the possession of the single 

 valve, were right in assigning it to China. Doubts arose of this from the fact that, 

 heretofore, Triquetra has been considered to be confined to South America. I propose 

 the name should be changed from that I originally gave it, {T. lanceolata,) as 

 inappropriate to the complete mature shell, to that of contorta, which is very descriptive, 

 aud can never be mistaken. 



