50 



Rhachura, a New Genus of Fossil Crustacea. 



Mr. William Gurley has recently sent me from the black limestone 

 of Danville, 111., a curious crustacean, allied to Dithyrocaris, con- 

 tained in a large kidney-shaped concretion. There are two separate 

 fragments, one partially overlapping the other; one is more conspic- 

 uous than the other, consisting of the last three segments of an 

 abdomen with a pair of lateral posterior appendages, ♦all of a dull 

 clay-color; while the other scarcely differs in color from the mat.ix, 

 being only the impression of a portion of a carapace, the edge of 

 which partially overlies one of the caudal appendages of the first 

 mentioned fragment. Were it not for its caudal appendages the 

 latter would resemble in a remarkable manner the thorax and abdo- 

 men of an insect, completely covered by its wings; for so closely do 

 the lines of ornamentation upon the penultimate and antepenulti- 

 mate segments resemble the veins of an insert's wings, that for a 

 long time I was inclined to consider them such, notwithstanding the 

 anomaly of the large exarticulate abdominal appendages. That it 

 should be found in the same nodule with the carapace of a fluviatile 

 or marine crustacean, seemed no more remarkable than the assem- 



