PLIOBOTHRUS. 57 



distinctly, but appeared to be a transverse slit in the middle of the 

 disk. It remained expanded but a short time. 



It is almost unnecessary to add, after these remarks, that the deep-sea 

 Milleporidae afforded no opportunity to observe their polyps. 



PLIOBOTHRUS Pourt. 



Corallum branching ; tissue more compact than in Millepora ; larger 

 pores scarcer, smooth, without any rudiments of septa ; smaller pores 

 tubulated, camenclryma finely porous, the pores linear and arranged in 

 rows. 



Pliobothrus symmetricus Pourt. 



Pliobolhrus symmetricus Pourt. Bull. Mus. Comp. Zocil , No 7. 



Plate IV., figs. 7 and 8. 



Corallum ramose, rising from an incrusting base and a short trunk, 

 branching into a more or less regular flabellum. Branches cylindrical, 

 flattened, and slightly expanded at the tip. The tendency in branching 

 is towards considerable symmetry between the two halves of the flabel- 

 lum. Three kinds of pores; very small, linear, over the whole csenen- 

 chyma ; larger tubulated, with very minute aperture when unbroken, 

 and larger round or oval ones scattered irregularly. Internal structure 

 coarsely porous; the larger pores expanding at the bottom into a flat 

 cavity communicating laterally with other canals. Tabulae very thick 

 and not numerous ; the interposed chambers soon filling up. Occa- 

 sional round cavities are found in the centre of the branch, filled with 

 a yolk-like substance contained in a membrane. 



Color gray. Height 5 to 7 cm. Diameter of branches about 5 mm. 

 Diameter of larger pores .4 to .5 mm. 



Off American Shoal, in 98 fathoms. 

 Off Alligator Reef, in 118 fathoms. 

 Off Sand Key, in 123 fathoms. 

 Off the Samboes, in 125 fathoms. 

 Off Sand Key, in 135 fithoms. 

 Off Sand Key, in 143 fathoms. 

 Off Sand Key, in 154 fathoms. 



It has also been found, according to Dr. Duncan, in from 500 to 600 

 fathoms, in the cold area to the northward of the British Islands 1 > \ the 

 '•Porcupine " Expedition. 



