266 Vniversitij of Calif ornia P ahlications in Zoology. [Vol.4 



tubercles which frequently coalesce, forming a broad calcareous 

 plate projecting above each zooecium; the aperture then seeming 

 to be guarded above and below, and flanked on each side by a 

 pair of projecting processes. Ooecia not known. 



This species is very characteristic and may be readily identi- 

 fied by its white calcareous incrustation investing the stems of the 

 common rockweed. The diagnosis given applies to the older 

 stages only. In different parts of the colony the zocecia present 

 very different appearances. The growing rim (pi. 15, fig. 16, 

 gr. r.) consists of a gelatinous mass divided by longitudinal lines 

 marking the lateral boundaries of the zooecia, each division con- 

 taining an apparently undifferentiated mass of cells, cl. m. Lines 

 at right angles to the lateral walls soon appear some distance be- 

 low the growing rim, marking off that portion into quadrangular 

 spaces and forming the proximal and distal walls of the future 

 zooecia. The lateral walls at the same time become distinctly 

 raised, and from their inner margin there grow at irregular in- 

 tervals, numerous irregularly shaped spinous processes, sp. pro., 

 some forked, others meeting across the intervening space. At 

 this stage, the aperture occupies the whole of the front, and 

 except for the irregular processes projecting from the walls, the 

 zowcia have an appearance much resembling M. memhranacea; 

 neither polypide nor operculum is yet formed, but in the interior 

 the mass of protoplasm, 2^>'o. m., destined to form the polypide 

 may be seen, and in the young zooecia below, the polypides, pd., 

 are found in various stages of development. At a slightly later 

 stage of growth (fig. 17) the lateral margins of the zooecia be- 

 come raised and the proximal and distal margins adjoining each 

 other become covered by a calcareous wall, cal. wa.; the outer 

 distal angles, ang., of each zooecium become much thickened and 

 show a tendency to project ; the aperture acquires a semi-circular 

 operculum consisting of a bar of chitin, and the polypide is 

 formed. In retraction, re. pd., this is drawn into the proximal 

 portion of the zooecium behind the calcareous wall. In the older 

 parts of the colony the calcareous deposition increases greatly re- 

 ducing the size of the aperture (pi. 16, fig. 18, ap.) ; the distal 

 angles thicken and project in two blunt processes or spines, sp., 

 which sometimes coalesce into a single bifid tubercle, or even into 



