202 Papers from the Marine Biological Laboratory at Tortugas. 



The ooecia are globose, raised, smooth, without pores; a thin extension 

 of the peristome runs upon the top and fuses with the outer layer of the 

 ovicell, leaving a thinner area upon the front. 



One small colony incrusting a shell at 8 fathoms. 



Arborella, new genus. 



Zoarium erect, dichotomously branched, with flexible corneous joints 

 at the bifurcations. Zooecia arranged in pairs, back to back, each pair 

 facing at right angles to the preceding pair of the series, giving a somewhat 

 quadrangular appearance. Front wall porous, but without special en- 

 larged pores. Fertile zooecia shorter than the infertile. Orifice with a 

 distinct sinus. Ovicell external, but inserted partly under the front of the 

 zooecium, its orifice closed by the zooecial operculum. Spines and avicu- 

 laria absent. A clear, chitinous, uncalcified ectocyst covers the zooecium 

 and ovicell. 



The relationships of this genus can not as yet be stated positively. 

 The general appearance of the colony and especially the character of the 

 joints and the membranous ectocyst seem to relate it to Tubucellaria, but 

 the character of the ooecium and the manner of origin of the zooecia at the 

 tips of the branches, together with the absence of a special median frontal 

 pore, distinctly separate it from this genus. It may be necessary to erect 

 a new family to accommodate it. Sectioned material will be studied to 

 determine the points necessary for a more complete understanding of the 

 relationships. 

 Arborella dichotoma n. sp. (Figs. I 3 to 15.) 



Zoarium erect, dichotomously branching, with hinged, flexible joints at 

 the points of bifurcation; internodes consisting of 8 to i8 pairs of zooecia, 

 each pair set slightly in advance and at right angles to the preceding pair 

 of the series, facing in four directions. Zooecia broad fusiform, wedge- 

 shaped at the base where they are inserted between the preceding pair. 

 The front wall evenly arched, cryptocyst well calcified, porous, with no 

 specialized, larger pores. Ectocyst uncalcified and covering the calcareous 

 wall as a clear layer. The orifice is evenly rounded in front and on the 

 sides, somewhat straighter on the posterior margin, with a broad, shallow, 

 but very evident sinus. The operculum is compound, well chitinized, with 

 an arched rib on either side, extending inward and upward from the hinge 

 denticle. Avicularia and spines are wanting. The fertile zooecia are some- 

 what different in form from the ordinary ones, being shorter and somewhat 

 broader, and the ooecium incloses nearly all of the orifice. The ooecium is 

 rounded in outline, large and prominent, the aperture looking upward and 

 closed by the zooecial operculum. The ooecial wall is porous like that of 

 the zooecium and is covered by the clear ectocyst. 



The colony was evidently broken off in dredging and no description can 

 be given of the base or the mode of attachment. 



