42 Benedict Arcturidse in the U. $. National Museum. 



truly says of deep-sea Isopoda : * "Although the number of deep- 

 sea species which have well-developed eyes is so large, they 

 nearly all (all except three) belong to the two allied genera Arc- 

 turns and Astacilla, which thus form almost the only exception 

 to the general statement that deep-sea Isopoda are blind." Of 

 Astacilla he says : f " Unlike Arcturus, Astadtta is almost exclu 

 sively an inhabitant of the shallow waters, only one species, in 

 deed, Astacilla granulata, ranging into deep water." 



KEY TO THE SPECIES OP Arcturus. 



a. End of the abdomen notched, as seen from above. 



b. Body smooth and free from spines beriitganus. 



b 7 . Body spiny. 



c. Head and six segments of the thorax each with a 



pair of spines on the dorsum longispinis. 



c,'. Head and segments of the thorax with not less 



than two pairs of spines to the segment, 

 d. Second and third articles of the antennae with 

 out spines except at the articulations hystrix. 



d 7 . Second and third articles of the antennae with 



spines on the bodies of the articles. murdochi. 



a 7 . End of the abdomen without notch. 



b. Thorax without spines above the epimera. 

 c. Abdomen acute or subacute at extremity. 



d. Eyes elevated on peduncles oculalus. 



d 7 . Eyes not elevated on peduncles. 



e. Extremity of abdomen notched in a lat 

 eral view. 



f. Thorax very tubercular stebbingi. 



f 7 . Thorax not tubercular abymcola. 



e 7 . Extremity of abdomen without notch, 

 f. Fourth segment of the thorax much 



longer than the preceding segments . glabrus. 

 f 7 . Fourth segment of the thorax but little 

 longer than the preceding segments, 

 g. Thorax with large swellings or 



tubercles tuberosus. 



g 7 . Thorax without tubercles myops. 



c 7 . Abdomen rounded at extremity. 



d. Abdomen notched at its extremity in lateral 



view spinifrons. 



d 7 . Abdomen without notch at its extremity. 



e. Epimeral spines present anna. 



e 7 . Epimeral spines wanting coppingeri. 



* Report on the Isopoda collected by the ' Challenger,' p. 166. 

 f Op. tit., p. 107. 



