NEW EXOTIC UNIONIDiE. 387 



Hah. — Buenos Ayres, South America, M. D'Orbigny. 



My cabinet. 



Diam. -6, Length 1-8, Breadth 1-6 inch. 



Shell smooth, obliquely oblong, flattened at the sides, and enlarged towards the 

 anterior margin, very inequilateral, acutely angular behind and round before ; sub- 

 stance of the shell rather thick, thicker before ; beaks prominent, with divergent coarse 

 undulations at the tips; ligament rather long, thin and light-brown; epidermis 

 shining, yellowish-green, with dark green transverse bands, without rays, and with 

 rather distant marks of growth ; umbonial slope raised into a somewhat acute angle ; 

 posterior slope rather narrow, somewhat flattened, and furnished with undulations to- 

 wards the beaks ; cardinal teeth large, trifid in both valves, sulcate and crenulate ; 

 lateral teeth long, crenulate, trifid in the right valve, and double in the left ; anterior 

 cicatrices small, well impressed, confluent with the lower, but distinct from the upper ; 

 posterior cicatrices well impressed and confluent ; dorsal cicatrices placed across the 

 centre of the cavity of the beaks; cavity of the shell shallow and wide; cavity 

 of the beaks shallow and rounded ; nacre silver-white and iridescent. 



Rernarlis.—k. single specimen was sent to me many years since by M. D'Orbigny 

 as a young Burroughianus (nobis), but I have always been satisfied that it was distinct 

 from that species of which I have a good suite of nearly all ages. It is certainly 

 closely allied to it, being of the same greenish color, with yellowish and green bands, 

 and nearly the same kind of large divergent undulations or costse at the beaks. It 

 differs, however, in being oblong, while the other is elliptical, and in the cardinal and 

 lateral teeth there is a very remarkable difference, the Burroughianus being of the 

 normal type, while this species has the aberrant form of being trifid. It is also flat- 

 tened on the sides, while the other is inflated there. There is another South Ameri- 

 can species which it reminds one of— the parallelqpipechn (nobis.) Like that species it 

 is green, is enlarged at the anterior region, and has transverse bands, but it is not by 

 any means so transverse, nor of so dark a green, nor does it agree in the teeth. The 

 trifidus more nearly resembles a young trapezialis (nobis), than any of our indigenous 

 species, the outline and flattened sides being somewhat alike, but it has no folds on 

 the sides nor is it so transverse. This specimen having been given to me among 

 other and various fine species by that distinguished and lamented naturalist, A. 

 D'Orbigny, collected by himself, during many years of perilous journeying in South 

 America, it would seem to be only due in courtesy to name it in honor of him, but 

 his name is already occupied by a species described by Deville and Huppe, from the 

 Upper Amazon. 



Unio piger. PI. 45, fig. 296. 



Testa tavi, elliptica, inflata, subaequilaterali, postic^ obtuse angulata, antice oblique rotundata; valvulis 

 crassiusculis, autice aliquantd crassioribus ; natibus subpruiuineutibus, inflatis, ad apices divaricate 



