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 

 Milleporidi^ afforded no opportunity to observe their polyps. 



PLIOBOTHRU.S Pourt. 



Corallum branching ; tissue more compact than in Millepora ; larger 

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

 tubulated, ca^neuchyma finely porous, the pores linear and arranged in 

 rows. 



Pliobothrus symmetricus Pourt. 



Plinholhriis sijmmclricus PouKT. Bull. Mus. Conip. Zool , 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 cJBnen- 

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

 and larger round or oval ones scattered irregidarly. Internal structure 

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

 cavity communicating laterally with other canals. Tabula; 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 jsores .4 to .T) mm. 



Oir Aiiu'rican Slioal, in 98 fotlioms. 

 Off Alligator Reef, in 1 IS fathoms. 

 Off Sand Key, in 123 fathcjins. 

 Off the Samboes, in 12.5 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 Island.s by the 

 "Porcupine " Expedition. 



