66 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



calice; it is rather thick and terminates in one or several prominent ascending 

 spines. 



The coenenchyma is usually very dense, its surface beset with isolated, erect, 

 compressed, truncated spinules. A circle of more prominent spinules surrounds 

 each calicular cavity, those intervening usually less prominent. 



The corallites cavities often solidly filled with internal deposit, rendering the 

 corallum very dense; tabulae, when present, from .5 to 1 mm. apart. 



Locality : — Shore, Easter Island. 



Remarks : — This is the most distinctly characterized species of Pocillopora 

 that has come under my observation, as I know no other that is really very 

 similar. The thick branched or frondose species in which the septa are well 

 developed and the columella prominent are P. modumanensis Vaughan 

 (Hawaiian Islands), P. ligulata Dana, P. plicata Dana, P. coronata Gardiner, 

 P. eydouxi M. Edw. and H. and P. elongata Dana. P. rugosa Gardiner has in 

 the lower calices a slender very prominent columella, but the septa are very 

 indistinct ; P. capitata Verrill, from Panama, in some of its variations appears 

 to have distinct septa and a styloid columella. The only species whose name 

 is listed above with which P. diomedeae need be compared is P. elongata Dana. 

 P. elongata has longer, thicker branches, more uniformly developed and 

 uniformly distributed verrucae, the columella (judging from Verrill's rede- 

 scription of the type) x is not so thick, and the texture much more porous. 

 It is very probable that the exsert margins of a portion of the septa in P. dio- 

 medeae and the compressed, truncated granulations of its coenenchyma consti- 

 tute additional differences. 



Bathyactis marenzelleri, sp. nov. 



Plate 4, Figs. 1-1 b. 



Base of the corallum circular, 22.5 mm. in diameter, almost flat, slightly concave 

 in the middle. The wall is extremely thin and translucent. Thin, slightly wavy 

 costae correspond to all septa, but become obsolete on the central portion of the 

 base ; those corresponding to the last cycle smaller and more irregular in develop- 

 ment. The costal edges are irregularly, sometimes coarsely, dentate. 



The calice is superficial, almost everted. Septa extremely thin, in four complete 

 cycles, forming six septal groups, one group between each pair of primaries. The 

 tertiaries fuse by a kind of calcareous membrane to the included secondary, and 

 the quaternaries fuse nearer the wall by their inner margins to the included ter- 

 tiary. There is an occasional rudiment of a fifth cycle. The margins of the 

 primaries are very tall, projecting 9 mm. above the base, the secondaries are 

 almost as prominent as the primaries, the tertiaries are slightly less prominent 

 than the secondaries, the quaternaries are decidedly less prominent than the other 

 septa. The outer portion of the septal margins is irregularly lacerate, the inner 

 half between the columella and the periphery possesses from three to four dis- 

 tant, tall, erect, thin, spines. Septal faces without granulations, fluted, with distant 

 carinae, some of which connect below with synapticula. These carinae vary from 



i Proc. Essex Inst., 6, p 99, 1869. 



