EEPOET ON THE ISOPODA. 107 



Astacilla, Fleming. 1 



Leac[h]ia, Johnston, Edin. Phil. Journ., vol. xiii. (1825) p. 219. 



Astacilla, Fleming, Encycl. Brit., ed. 7, vol. vii. p. 502. 



Arcturus, Guerin-Meneville, Iconogr. d. Eegne anim., t. xxxi. fig. 2, 1829-44. 



1 have already (p. 85) stated the reasons which necessitate the separation of this genus 

 from Arcturus. 



Unlike Arcturus, Astacilla is almost exclusively an inhabitant of the shallow waters, 

 only one species, indeed, Astacilla granulata, ranging into deep water. The genus appears 

 to be almost world-wide ; it is known from the European and American coasts, from the 

 Cape, from Australia and New Zealand, and in the present Eeport I have a new species 

 to describe from Kerguelen. 



Astacilla marionensis, n. sp. (PL XXV. fig. 5). 



A single specimen dredged in 100 fathoms, off Marion Island, is the sole representative 

 of this species, which does not present any very salient characters. 



Its length is 8 mm. exclusive of the antennae. 



The head is notched anteriorly in the middle line ; laterally and below it is prolonged 

 into a short triangular process ; the upper surface is very convex between the eyes, the 

 separation between the head and the first segment of the thorax, which is fused with it, is 

 marked by a furrow ; this segment and the two following are subequal ; the second and 

 third segments have a pair of dorsal tubercles, one close to the median dorsal line, the 

 other about half way between this and the lateral margin of the segment, the more dorsal 

 tubercles are distinctly larger than the lateral ones ; the first segment of the thorax 

 appears to have a smaller unpaired median tubercle ; the fourth segment of the thorax 

 is greatly elongated, equalling in length the head and first three segments ; it has ten or 

 twelve tubercles arranged in pairs on either side of the dorsal median line ; the posterior 

 extremity of the segment is marked by a prominent hump on the dorsal surface. 



Of the three posterior segments of the thorax the first is rather the largest ; the anterior 

 part of each segment is flattened, the posterior part is raised into a prominent ridge which 

 occupies laterally the whole of the segment. On the first segment are a pair of prominent 

 tubercles on either side ; the most anterior of these is directed forwards ; the posterior 

 tubercle is more dorsal in position and directed upwards ; these tubercles appear in the two 

 succeeding segments to be represented by only one in the lateral region, which is much 

 smaller ; the epimera of the fifth segment are very much deeper than in the succeeding 

 segments. The first two segments of the abdomen are distinctly separate, the third 

 practically fused with the caudal shield ; the first segment is smooth, the second has a pair 

 of tubercles, one above the other, on each side of the body. The caudal shield has the 



1 I have followed Harger (Marine Isopoda of New England, U.S. Commission of Fish and Fisheries Report for 1878, 

 Washington, 1880, p. 363), in retaining the name Astacilla. 



