POLYCH.ETA FROM NORTH AMERICA. 801 



thick and opaque, slightly roughened, but without definite growth- 

 lines. A slight median ridge and sometimes one on either side ; 

 the aperture has, however, an entire margin and measures 2 mm. 

 across. 



The animal is of a uniform brick-red colour, and its total 

 length is 4*5 mm ; there are 14 branchiae — 7 being joined at 

 their bases and situated on the right side ; the other 6 func- 

 tional ones are joined with the operculum on the left. Each 

 rachis has a thin membranous projecting flap which wraps round 

 the outside of the next, and thus gives rise to a series of imbri- 

 cating semilunar membi-anes just inside the base of the collar. 

 The total height of the gills is 1 mm. There is a very wide 

 thoracic membrane on the right side which almost envelops the 

 whole animal. The calcareous plate of the operculum is 1'5 mm. 

 along its long axis ; the outer side is concave and generally 

 covered with sand ; the talon projects obliquely inwards and has 

 large wing-like expansions at the sides (figs. 14 c & d). The 

 collar setae (fig. 14 a) appear to be in an intermediate stage between 

 plain bladed form and that with a distinct fin-like expansion at 

 the base of the blade — in one specimen I could find none of these, 

 only forms with the ordinary blades. In the 2nd segment there 

 were numerous setje of the normal kind, and the 3rd fascicle was 

 made up of bladed sickles (fig. 14 e) and ordinary bladed setae. 

 The two tori in the thorax consisted of uncini of the ordinary 

 shape with about 20 teeth. In the abdomen were counted 20 to 

 25 segments, and the setae were of the ordinary geniculate type. 



15. Spirorbis laxgerhaxsi Caullery & Mesnil (5), 1897, and 

 Bush (3), 1905, from Panama. (PI. LXXXIX. fig. 15.) 



Specific characteristics : — • 



1. Collar setfR with fin-like expansion at base of coarsely 



serrated blade (fig. 15). 



2. Embryos incubated in operculum. 



3. Operculum of characteristic shape (5. fig. 22). 



On tubes of ,SerpuJa cohmihiana from Departure Bay. Tubes 

 small, marked with lines and ridges. 



The structure of the animals agrees with the description given 

 by Caullery & Mesnil (5. p. 217). They are 1-2 mm. "long, 

 the abdomen being broad, with a long asetigerous region followed 

 by 9 short setigei'ous segments. 



16. 



Spirorbis morchi Levinsen (10), 1883 (Greenland); Caullery 

 & Mesnil (5), 1897; Bush (3), 1905. (PI. LXXXIX. 

 figs. 16«-16fZ.) 



Specific characteristics : — • 



1. Collar setae about 12, with fin-like expansion at base of 



coarsely sen-ated blade (fig. 16«). 



2. Embryos incubated in operculum (fig. \%h). 



3. Branchiae 8. 



