ISOPODS OF NORTH AMERICA. 327 



as wide at its anterior end as it is at its posterior extremity; it 

 measures 2 mm. in width at the anterior portion and only 1 mm. at the 

 posterior end. 



The abdomen is composed of two segments, the first one of which is 

 short and narrow. The terminal segment has the lateral margins pro- 

 duced on either side in two angular processes, one at the base of the 

 segment and the other a little below the middle. The segment termi- 

 nates in an acute point. There is also a median tubercle on each one 

 of the abdominal segments, the tubercle on the terminal segment being 

 situated at the base of the segment. 



The marsupial plates are covered with small tubercles. 



S3. Genus ARCTURUS Latreille. 



Body slender, somewhat cylindrical in form, with the fourth seg- 

 ment of the thorax not greatly longer than the others. Epimera small 

 but distinct on all the thoracic segments with the exception of the 

 first. Marsupium of female composed of three pairs of plates issuing 

 from the second, third, and fourth segments. Abdomen composed of 

 three segments, two segments anterior to the large terminal one. 



ANALYTICAL KEY TO THE SPECIES OF THE GENUS ARCTURUS. 



a. End of terminal segment notched, as seen from above. 



b. Body smooth and free from spines Arcturus beringanux Benedict 



b' '. Body spiny Arcturus longispinis Benedict 



a'. End of terminal abdominal segment without notch. 



6. Body without spines or tubercles, perfectly smooth and glabrous. 



Arcturus glaber Benedict 

 I/. Body with spines or tubercles. 



c. Terminal segment of abdomen armed with a long median terminal spine, pro- 

 jecting beyond the end of the segment. 



d. Head with two spines. Second joint of second pair of antennae armed with 

 one spine at upper end. Thorax with few spines. Surface of terminal 

 abdominal segment smooth. 



e. Second joint of peduncle of second pair of antennae without spine at base 

 on outer margin. First pair of antennae extending one-third the length 

 of the third joint of second pair of antennae. Dorsal spines wanting on 

 second abdominal segment. Spines wanting on opercular valves. 

 Anterior thoracic appendages furnished with a number of spines on 



the proximal joints Arcturus purpureus Beddard 



ef. Second joint of peduncle of second pair of antennae with spine at base on 

 outer margin. First pair of antennae extending two-thirds the length 

 of the third joint of second pair of antennae. Dorsal spines present on 

 second abdominal segment. Spines present on opercular valves. 

 Anterior thoracic appendages without spines except on penultimate 



joint Arcturus caribbseus Richardson 



d f . Head with eight spines. Second joint of second pair of antennae armed 

 with three spines at the upper end. Thorax with many spines. Sur- 

 face of terminal abdominal segment with three rows of spines on dorsal 

 surface. Row of spines on each opercular valve. 



Arcturus floridanus Richardson 



