NEW EXOTIC (JNIONID^E. 391 



Dl ^- 1-3 ' Le "§th 2-1, Breadth 34 inches, 



bhell smooth, elliptical, inflated, inequilateral, subbiangular behind, regularly 

 rounded before ; substance of the shell thick, somewhat thicker before, with a 

 deep red, broad margin on the inside; beaks slightly prominent, smooth at the tips 

 being entirely without undulations; ligament rather large, not prominent; epider- 

 mis cinnamon-red, lighter towards the beaks and much darker at the margin, with 

 distant marks of growth, without rays or very obscurely rayed; umbonial slope 

 flattened; posterior slope carinate, with two obscure, colored lines on each valve 

 one of which follows a slightly-impressed line; anterior cicatrices distinct and well 

 impressed; posterior cicatrices confluent and slightly impressed; dorsal cicatrices 

 apparently none; palleal cicatrices slightly impressed and distant from the margin- 

 cavity of the shell rather deep and wide; cavity of the beaks shallow and sub- 

 angular; nacre of a fine light rose color,-the broad, deep red margin presenting a 

 deep contrast, — beautifully iridescent. 



Soft Parts.-Bramhial uterus not charged, but. minute ova were found in one of 

 the only two received in alcohol. Branchice large, nearly semicircular, all very 

 nearly the same size, united the whole length of abdominal sack. Palpi rather 

 large, round, united only at the upper part. Mantle rather thin in the male speci- 

 men and thick in the female, thickened at the edges, with numerous papilla, below 

 the branchial opening. Branchial opening large, with numerous small dark-brown 

 papilla, on the inner edges, inside of which the edges are nearly black. Anal open- 

 mg large, with a dark edge not furnished with papilla.. Super-anal opening lame, 

 not united below. Color of the mass dirty white. 



Pemarhs.-This very beautiful species belongs to the group of which An. Wheat- 

 kyi, Lea, may be considered the type. It differs from that species in being larger, 

 more ponderous, having a greater axis minor and being of a darker red. It has 

 the usual triangular sinus at the end of the ligament,-a character prevailing with 

 the South American species. The smaller of the anterior cicatrices is unusually 

 proportionately small. Most of the specimens before me, which consist of a good 

 suite, present an obscure line on the middle, from the beak to the base, posterior to 

 which the epidermis is slightly darker. The impressed line on the posterior slope 

 causes a slight emargination on the posterior margin. This is certainly one of the 

 most beautiful of the genus, and I have great pleasure in dedicating it to Prof. 

 Jeffries Wyman, of the University of Cambridge, Massachusetts. It was captured 

 in the Uruguay Eiver by him, with many other fine Unionidce, many of which 

 have not before been observed. 



A very remarkable character in the round form of the palpi is found in this spe- 

 cies and in An. ialo-margiuata, (nobis.) They differ in this from all North American 

 species I have examined, and it is a very interesting point to ascertain if this be the 



