87 



TRIARTHRUS BECKII. Green. Cast No. 34. Fig. 6. 



Cauda subrotunda, bipunctata; articulis abdominis 

 tribus, absque lobis lateralibus consuetis, sed lobo 

 arcuato utrinque apposite. 



The only portions of this fossil which have yet been 

 found, are the abdomen and tail. The abdomen is 

 composed of three joints; the first passes from the 

 side lobes completely over the body, and on its upper 

 surface, near the middle of the back, there is often a 

 minute elevated pimple. The other two, pass ob- 

 liquely from the lateral lobes, and are interrupted in 

 their course over the body. The tail, or posterior 

 portion, is expanded, something like that of the Iso- 

 telus or Dipleura, and has a deep puncture on each 

 side, about half the distance between its terminal 

 border and the last articulation of the abdomen. The 

 lateral lobes are unlike those of any other genus. 

 They form narrow cuniform appendages to the sides; 

 near the first joint of the abdomen they are crossed 

 transversely by an elevated ridge, from which they 

 gradually taper along the sides of the body, and ap- 

 pear to inosculate in a delicate point at the central 

 border of the tail. The abdominal articulations do 

 not pass over these lobes, but just below the last 

 joint, a little transverse furrow, in perfect specimens, 

 may be noticed. The largest specimen of this frag- 

 ment I have seen is exactly half an inch in length. 

 This fossil occurs in black shaly limestone, on the 



