47 26 Proceedings of the Royal Irish Academy. 



The known species may be separated as follows : — 



Compound setae with winged tips = S. arenae Webster and Benedict. 

 Compound setae without winged tips = a. 



Thick spines present in 23 anterior segments = S. varians "Webster 



and Benedict. 

 Thick spines present in 5-6 anterior segments = b. 

 , I Thick spines in setigerous segments 2-5 = c. 



I Thick spines in setigerous segments 2-6 = & bidentata sp. n. 



( Spine inlst setigerous segmentveiy thick = S. "varians" de St.-Joseph. 



\ Spine in 1st setigerous segment slender = S. Wcbstcri sp. n. 



The genus Syllides is very close to Streptosyllis, differing only in the 

 apparent absence of any differentiation between the anterior and posterior 

 parapodia. In Syllides Verrilli Moore (1907, p. 448), the end of the shaft 

 of the seta approaches the condition characteristic of Streptosyllis. 



Streptosyllis Websteri sp. n. 

 PL II, figs. 3a-f. 



This species, which is named after one of the American naturalists who 

 created the genus Streptosyllis, was taken on several occasions in the harbours 

 of Bonn and Ballynakill. All the specimens were sexually mature, provided 

 with swimming setae, and were taken in the tow-net, usually at the surface, at 

 night. No specimens belonging to this species have been yet found in the 

 bottom fauna, and consequently their habitat is not known. 



The worrus are 3'5-5 mm. long, consisting of 32-49 setigerous segments. 



The head (fig. 3a) is broader behind than in front, with four large 

 circular reddish-brown eyes provided with lenses. The median antenna 

 is missing. The lateral antennae, rising from the front of the head, are 

 smooth and cylindrical, slightly narrowed at the base. The palps are small 

 and filiform, ventral in position. Between the head and the buccal segment 

 on each side are the ciliated nuchal organs. The buccal segment bears on 

 each side a pair of smooth tentacular cirri, longer than the lateral tentacles, 

 but shorter than the anterior dorsal cirri. All the cirri are filled with 

 rounded faintly yellow granules. The anal segment (fig. 3b) bears two short 

 lateral cirri, and a long median one. 



The feet are prominent. The anterior region of the body consists of 

 5 setigerous segments, as in the " S. varians " of de Saint-Joseph, but differs 

 from that species in that the spine in the first segment is thin, resembling 

 that in the posterior segments. In segments 2-5 the spine is very large 

 and thick (fig. 3c). 



