288 BULLETIN 54, UNITED STATES NATIONAL MUSEUM. 



ANALYTICAL KEY TO THE SPECIES OF THE GENUS EXOSPHSEROMA. 



a. Body widening gradually from the head backward. Thorax transversely ridged 

 and provided with three longitudinal rows of small tubercles. Branches of 



uropoda very large, expanded Exosphseroma, amplicauda (Stimpson) 



a'. Body not increasing in width from the head backward. Surface of thorax 



smooth. Branches of uropoda not expanded. 

 6. Extremity of terminal abdominal segment produced in a rhomboid process. 



Exosphseroma rhomburum (Richardson) 



b / . Extremity of terminal abdominal segment not produced in a rhomboid process. 

 c. Surface of abdomen with tubercles. 

 d. Abdomen posteriorly ending in a triangular apex, on either side of which is 



a small tooth. 



e. With three tubercles at base of terminal abdominal segment. Outer 

 branch of uropoda not longer than inner branch and both shorter than 

 terminal segment of abdomen. Outer branch not acutely produced. 



Exosphseroma yucatanum (Richardson) 



e! '. With two tubercles at base of terminal abdominal segment. Outer 

 branch of uropoda extending beyond the inner branch, a distance equal 

 to half the length of the inner branch. Outer branch very acute at 



extremity Exosphseroma faxoni, new species 



d / . Abdomen posteriorly rounded. With eight tubercles, six on terminal seg- 

 ment and two on preceding segment. 



Exosphseroma octoncum (Richardson) 

 c 7 . Surface of abdomen without tubercles. 



d. Outer branch of uropoda smooth on exterior margin. 

 e. Outer branch of uropoda half as long as inner branch and half as wide. 



Exosphseroma Ihermophilum (Richardson) 



^. Outer branch of uropoda more than half as long as inner branch. 

 /. Outer branch four-fifths as long as inner branch and half as wide. 



Exosphseroma dugesi (Dollfus) 

 /'. Outer branch two-thirds as long and as wide as inner branch. 



Exosphseroma oregonemis (Dana) 

 d'. Outer branch of uropoda crenulated on exterior margin. 



Exosphseroma crenulatum (Richardson) 



EXOSPHSEROMA AMPLICAUDA (Stimpson). 



Sphseroma amplicauda STIMPSON, Bost. Jour. Nat. Hist., VI, 1857, p. 510. 

 RICHARDSON, Proc. U. S. Nat. Mus., XXI, 1899, p. 835; Ann. Mag. Nat. 

 Hist. (7), IV, 1899, p. 179; Amer. Naturalist, XXXIV, 1900, p. 222. 



Localities. Tomales Ba}^, California; Kyska Harbor, Alaska; north 

 coast of Amchitka, Alaska; Monterey Bay, California. 



Found in low water on beach. 



Body narrower anteriorly than posteriorly, gradually increasing in 

 width from the head to the abdomen, a little more than one and a 

 half times longer than broad, 5 mm. : 8 mm. 



Head a little more than twice as wide as long, with a frontal border 

 arising between the eyes, which is produced in a small median point. 

 The eyes are small, round, composite, and situated in the post-lateral 

 angles of the head. The first pair of antennae have the basal article 

 long and equal in length to the third; the second article is half as 

 long as either of the other two. The flagellum is composed of five 



