1908.] 



NATURAL SCIENCES OF PHILADELPHIA. 



363 



At the same time the tube, which is perfectly smooth in the 

 early stages, becomes roughened by growth lines, and its walls become 

 very thick, solid and stony, and are ornamented by three thick and 

 stout ridges rounded on the free side and covering most of the outer 

 surface of the shell. Here and there the depressions between them are 

 crossed by transverse spurs and rods. At the aperture of the tube 

 these ridges project as three very strong and prominent teeth. Fully 

 developed tubes are usually 3.5 mm. in diameter and composed of 4 

 to 4J turns. The carinae begin at the end of the third turn and I.evin- 

 sen's figure very accurately represents one in a half-grown condition, 

 in which the ridged whorl is just beginning to turn in upon the inner 

 coils. One more turn, with the ridge characters exaggerated, would 



Spirorbis tridentatus — a, an operculum in side \\eyv, filled -vvith embryos and 

 showing the imperfect four-tiered calcareous plug, X 24; fe, one of the calcare- 

 ous plates detached and seen from the inner surface, X 24; c, a collar seta, 

 the fin at the base may be somewhat too long, X 600; d, the two seta? of an 

 abdominal bundle, X 600. 



result in a condition exactly like my full-grown tubes, in which the 

 inner coils are completely concealed from above and the exposed parts 

 bear massive ridges. Where free to grow without restraint the tubes 

 are strictly discoid and the lower surface of all of the coils is in intimate 

 contact with the alga to which they are attached, but when the indi- 

 viduals are crowded the coils are heaped up in various irregular and 

 often angulated forms. 



In general form the operculum (a) agrees well with that of S. granu- 

 latus, being a slender cone containing a broad pouch filled with embryos 

 and tapering regularly into a long but rather stout stalk. The cal- 



