396 SPIRORBIS MEDIUS. 
The Spirorbis Beneti of Marion’ approaches this form closely. 
In Cunningham and Ramage’s (1888) figure three pairs of setigerous processes sym- 
metrically placed occur on the anterior region. ‘The structure of the first group is, however, 
recognisable. 
Miss Bush (1905) states that the terminal blades of the first bristles in the American 
forms are finely serrate; indeed, in some they are scarcely visible. 
Some are inclined to lmk on the true Sprrorbis Pagenstechert from the Mediterranean 
to this species, but the southern form develops ova in its operculum, which the northern 
does not. Claparéde and Mecznikow, however, mention that the species, which they term 
S. pagenstechert, did not develop ova in the operculum, so that further investigation is neces- 
sary. Lo Bianco (1909) found the Neapolitan species with ova and larve from June to 
September. 
3. SPIRORBIS MEDIUS, Prvell, 1912. Plate CX XXIII, figs. 3—3c—bristles and hooks. 
Specific Characters.—Collar typical. Branchiz fourteen, seven joined at their bases on 
the right side and six besides the operculum on the left. Hach filament has a thin mem- 
branous flap, which overlaps the next, thus forming a series of imbricate semilunar membranes 
inside the base of the collar (Pixell). Body with a wide thoracic membrane on the right side 
which almost envelops the whole animal. Calcareous plate of the operculum is nail-shaped 
and 1°5 mm. in its long axis, the free surface being concave. The talon projects obliquely 
and has large wing-like expansions at the sides. The collar-bristles are intermediate between 
simple serrated blades, and those with a gap and fin. The second bristles are simple, rather 
broadly winged forms, and, besides these, in the third are sickle-shaped forms with distinctly 
serrated tips. Abdominal bristles geniculate with boldly serrated edges. Thoracic uncini 
triangular with about twenty fine teeth, the great fang being long and clavate, and with a 
narrow gulf behind it. Tube large, flat, thick and opaque, slightly roughened, but without 
definite lines of growth. A slight median ridge or sometimes one on either side. Aperture 
entire (Pixell). 
SYNONYMS. 
1912. Spirorbis medius, Pixell. Proc. Zool. Soc. Lond., p. 800. 
1914. 3 » Southern. Proc. Roy. Irish Acad., vol. xxxi, No. 47, p. 149. 
1915. - » idem. Irish Se. Invest., No. 3, p. 49. 
Habitat.—Blacksod Bay, in dense masses, under stones between tide-marks (Southern). 
Hlsewhere it occurs in British Columbia (Pixell). 
This species of Spirorbis was originally described by Miss Pixell from the shores of British 
Columbia. Mr. Southern lately procured it in Blacksod Bay on the West Coast of Ireland, 
and I am indebted to him for examples. 
The cephalic collar is well developed, forming a sheath for the base of the tentacles. 
The filaments of the branchiz, which are fourteen in number, terminate distally in a tapering 
subulate process, which extends considerably beyond the pinne. The nail-shaped operculum 
1 «Ann, Se. nat.,’ 6° sér., t. vill, p. 29, pl. xvu, fig. 8, 
