Arcturidse in the U. S. National Museum. 43 



Thorax with spines above the epimera. 



c. Spines present in front of the ocular space. 



d. Spines, spinules, or spiny tubercles very nu- 

 merous on the thorax. 



e. Spines all long and slender multispinis. 



e'. Spines all short or with a few long ones, 

 f. All spines short. 



g. Third segment of antennae spinu- 



lose furcatus. 



g'. Third segment not spinulose. . . . glacialis. 

 i' . Spines long and short. 



g. With three spines extending back 



from the abdomen spinosns. 



g'. With two spines extending back 



from the abdomen americanus. 



d / . Spines of the thorax comparatively few. 



e. Last segment of the abdomen with a cate- 

 nate median line, 

 f. Second segment of abdomen with 



spines cornutus. 



f '. Second segment without spines tenuispinis. 



e'. Last segment without carina. 



f. Abdomen armed with a long median 

 spine which projects beyond the 

 end of the segment. 

 g. Upper surfaceof abdomen smooth . purpureus. 



g'. Upper surface spinulose studeri. 



F. Abdomen without median spine brunneus. 



c'. Spines absent in front of the ocular space. 



d. Head free from spines feiideni. 



d / . Head with spines present be; ween the eyes. . baffini. 



Arcturus baffini (Sabine). 



Idothea baffini Sabine, Appendix to Parry's First Voyage, p. 50, pi. i, 



figs. 4-6, 1824. 

 Arcturus tuberculatus Latreille in Cuvier, Regne Animal, ed. 2, IV, p. 139, 



1829. 

 Arcturus baffini Westwood, Trans. Entom. Soc. Lond., vol. I, p. 72, 1836. 

 Milne-Edwards, Hist. Nat. Crust., Ill, p. 123, pi. xxxi, fig. 1, 1840. 

 Sars, Crust. Norw. North Atlantic Expd., p. 97, pi. ix, figs. 1-21, 



1885. 

 Beddard, Report on the Isopoda collected by the ' Challenger,' pi. 

 xx, fig. 12, 1886. 



The best figures of this species are, in my opinion, those of Professor 

 Sars. It is the oldest and best known species of the genus, and lias been 

 taken over a larger range than any other. Its characters are so well 

 marked that it can be readily separated from any other species in the 

 collection. 



