BUS 



252 BUSH 



often unequal, spiral threads or carinae, the interspaces crossed by 

 numerous raised transverse lines which extend over the side, and in 

 fully developed specimens spread around the base. The operculum is 

 furnished with a peculiar calcareous cylinder in which well-developed 

 embryos, some with good sized setae, have been found. Some speci- 

 mens collected at Bermuda in February 1904, by Mr. Dwight Blaney, 

 have two complete cylinders, one above the other, on the operculum ; 

 others have a single large cylinder filled with well-developed eggs. 

 All the thoracic setas have narrow tapered blades. 



Spirorbis mutabilis sp. nov. 



Smaller, more or less regularly coiled sinistral tubes are found on 

 various shells at Bermuda, often with the preceding species. 



The surface is usually but little roughened, but sometimes very faint 

 spiral lines occur, and in rare instances, when the development has not 

 been impeded, the surface is ornamented with two keels which define 

 the flattened top, giving a four-sided appearance ; sometimes the aper- 

 ture is turned upward. The operculum is furnished with a thin, more 

 or less concave calcareous plate with small base. Some of the oper- 

 cula were filled with globular masses and others were of the ordinary 

 form. In some instances egg-chains were found in the tubes along the 

 dorsal furrow. The collar seta? have long, tapered, coarsely serrate 

 blades with conspicuous fin-like bases. 



NOTES ON THE GENUS SPIRORBIS, WITH A LIST 

 OF DESCRIBED SPECIES. 



The genus Spirorbis seems to have been purposely avoided by most 

 authors, little systematic work having been done since Morch, in 1863, 

 published the descriptions, with added notes, of all of the earlier 

 described species, straightening out much of the confusion in their 

 synonymy. 



Levinsen, in 1883, was the first to make a thorough study of the 

 northern species, by dissecting the animals, and has, by his excellent 

 figures of their tubes and important collar setae, done much toward 

 rendering it possible to correctly identify them. 



As little had been published in regard to the importance of the 

 operculum, with its protective calcareous plate, in connection with the 

 writer's study of the Alaska species, the animals of numerous Atlantic 

 forms found along the coast from Greenland to Bermuda have been dis- 

 sected with special reference to this character. The investigations were 



