POLYCH^TA. 261 



parts which has taken place, for example in such genera as Thalenessa and Iphione. 

 In the former the palps are concealed below the cirrophores of the tentacular cirri 

 (Plate II., fig. 52) ; in the latter the cirrophores of the tentacular cirri are concealed 

 below the palps (Plate I., fig. 6). 



Thalenessa stylolepis, n. sp. Plate III., figs. 53 to 56. 



Localities: Station LVIL, llj to 36 fathoms; and Station LVL, Dutch 

 Modragam Paar, 9 fathoms ; specimen incomplete behind ; taken out of coral block. 



This must be the worm referred to on p. 74 of the ' Ceylon Pearl Oyster Report,' 

 Part I., as Sigalion mathildee.* 



The chief characters, which typify a new section of the genus Thalenessa, are the 

 possession of a pair of small frontal antennae and an equally small median occipital 

 antenna ; the insertion of the elytra upon high peduncles ; and, in the anterior region, 

 the absence of the cirriform branchiae from the scaleless segments. 



Prostomium large, flattened, shield-shaped, with three notches or emarginations, 

 two at the frontal border from which the paired antennae arise, one at the occipital 

 border from which the tentaculum impar arises (Plate III., fig. 53). Only one pair 

 of eyes was observed occupying a central position behind the frontal emarginations. 

 The cirrophores of the buccal segment are porrect, sub-median and adnate to the 

 prostomium. The line of division between the frontal border and the cirrophores is 

 at the same time the line of concrescence of these structures, so that the antennae are 

 virtually inserted into the base of the cirrophores, a condition leading to that found 

 in Sthenelais, Sthenolepis and Psammolyce. The palps are long and smooth. 



The elytra are pedunculate, the elytrophores rising like stout pillars from the 

 dorsum, as in Eulepis ; the elytra are firmly attached to the elytrophores ; proceeding 

 outwards from the latter below each elytron is a rather long cirriform appendage 

 which does not occur on the intervening segments (segmenta nuda) ; the latter show 

 a small tubercle in the line of the elytrophores. The elytra are placed on the usual 

 segments up to the 27th, when they begin to be continuously successive. They 

 carry 12 to 13 (or fewer) plumose fimbriae (Plate III., fig. 56), which project straight 

 outwards from the outer border, followed by a few simple filiform papillae. The 

 elytra of the first pair are smaller than the following, rounded, not accompanied by 

 cirriform appendage. The remaining elytra cover the dorsum, their inner borders 

 meeting so as to form a tunnel along the length of the body ; on the inner side of 

 each elytrophore a small ctenidium projects into the tunnel, a pair of these ctenidia 

 being inclined towards one another in each body-segment (Plate III., fig. 54). As 

 mentioned above, the cirriform branchiae of Thalenessa stylolepis are confined to the 

 elytrophores. The dorsal ramus of the parapodium (Plate III., fig. 54) carries a 

 bundle of numerous long simple fringed setae, the fringes appearing in side view as a 



* The plumose fimbriae of the elytra exactly resemble those of Sigalion Mathilda' as figured by Professor 

 McIxtosh (1900). 



