310 ISOCIRRUS PLANICEPS. 



Genus CXX. — Isocirrus, Arwidsson, 1906. 



The nuchal grooves parallel almost to the front, where they widen. A well-marked 

 cephalic rim, which fails posteriorly, notched laterally. Few naked segments posteriorly 

 in front of the funnel, the edge of which is notched. Ridge of the ventral nerve-cord 

 distinct. Well-marked bands of glands in all the rings in front of the feet and also on 

 the dorsum. 



The anterior bristles have narrow wings and smooth or almost smooth points. Behind 

 these are bristles with stouter shafts, straighter tips, and broader wings. The posterior 

 bristles have narrow wings and long points. Shaft of the hooks of moderate length, hairs 

 on the throat at main fang. Tube free, straight. 



1. Isocmiius planioeps, Sars, 1872. Plate XCIX, figs. 5, 5 a, and 5 b — head and anal 

 region; Plate OVIII, fig. 4 — hook; Plate CIX, figs. 6 and 6 a — spine and hook. 



Specific Characters. — Cephalic plate resembling that of Praxillella prwtermissa, but 

 the notches are less deep, and the margin is crenate. Body typical ; twenty-five bristled 

 segments ; glandular anteriorly ; three segments with slightly curved spines ; fourth and 

 fifth segments have modified hooks. Funnel with twenty-seven nearly equal short conical 

 cirri. Ring from which it arises massive, and forming a shelf all round, so that the funnel, 

 as it were, springs from a stalk. Nephridia in bristled segments 6 to 9. Anterior 

 glandular belts to the eighth segment inclusive. Bristles of two kinds — strong with 

 tapered tips and wings, and slender forms with pinnate or spiked tips. Typical hook 

 with rather short, curved shaft, well-marked elliptical shoulder, neck broad distally, the 

 main fang having a semicircular notch below it ending in an eminence for the gular 

 bristles. Main fang makes nearly a right angle with neck, and has five to six teeth on 

 the crown behind it. 



Synonyms. 



1856. Clymene sp., Sars. Fauna Lit. Norveg., p. 17, Tab. 2, fig. 27. 



1872. ,, planiceps, idem. Annel. Christiania, p. 411. 



1900. Euclymene „ Verrill. Trans. Conn. Acad. Arts Sc, vol. x, p. 655. 



1906. Isocirrus „ Arwidsson. Skand. u. arkitsch. Maldan., p. 137, Taf. iii, figs. 98—107; 



Taf. viii, figs. 276—280; Taf. xi, figs. 348—351. 

 1913. Isocirrus D, Mcintosh. Ann. Nat. Hist., ser. 8, vol. xi, p. 94. 



Habitat— Dredged by Dr. Gwyn Jeffreys, in 1868, 35 miles off Skerries, Shetland; 

 and in 78 fathoms off the Hebrides, July, 1866. 



Abroad it occurs in the Norwegian fjords, off Sweden, and the Cattegat. It ranges 

 also to the Gulf of St. Lawrence, Canada. 



So far as a softened though large example from the Hebrides shows, the cephalic 

 plate, while having a general resemblance to that of Praxillella prxtermissa, differs from 

 it in the crenate condition of the margin and in the shallow nature of the notches, both 

 lateral and posterior ; yet in some large examples of P. pvdetermissa a crenate margin is 



