Plexaura Duchassaing et Michelotti. Coralliaires des Antilles, 1860, p. 25. 



Plexaura Kolliker. Icones Histologicae, II, 1865, p. 138. 



Plexaura Wright and Studer. Challenger Reports, the Alcyonaria, 1889, p. 137. 



Plexaura Thomson and Henderson. Pearl Oyster Fisheries Reports, the Alcyonaria, 1905, p. 304. 



The original describer, LAMOUROUX, gave a definition for this genus that needs little 

 change to fit it for modern use. It is as follows : 



"Polypier dendroide, rameux, souvent dichotome ; ramaux cylindriques et roides; axe 

 legerement comprime; ecorce, dans 1'etat de dessication, subereuse, ou terreuse, tres-epaisse, 

 faisant peu d'effervescence avec les acides, et couverte de cellules eparses, grandes, nombreuses, 

 souvent inegales et qui ne sont jamais saillantes". 



Eliminating non essential points, the difinition used for the present work will be as follows: 



Coenenchyma thick, calyces included and distributed on all sides of the stem and branches. 

 Spicules in the form of warty spindles and clubs, usually in two layers. Axis entirely horny, 

 except at the base of the stem of large specimens. 



The character of the axis will differentiate this genus from Plexaurella, which greatly 

 resembles it superficially, and from Euplexaura and Pseudoplexaura. The characters of the 

 spicules will separate it from Plexauroides and Hicksonella. 



The type of this genus is Plexaura heteropora (Lamarck). 



Owing to the superficial resemblance between Plexaura and Plexaurella, both genera 

 abundant in species, it is impracticable, without recourse to the types, to tell which of the 

 species originally .described should be placed in Plexaura. Both of these genera are but sparsely 

 represented in previous collections from the Western Pacific, or Indo-Pacific region. Hence it is 

 not surprising that the species in the Siboga collection are all apparently new. 



i. Plexaura aggregata new species. (Plate I, figs, i, i#; Plate IV, fig. i). 



Stat. 58. Anchorage off Seba, Savu Island. Up to 27 meters. 



Stat. 65". 7o'S., 1 20 34'. 5 E. Flores Sea. Depth changing rapidly from 400 120 meters. 



Coral bottom. 



Stat. 213. Saleyer anchorage, and surroundings. Up to 36 meters. 

 Stat. 258. Tual anchorage, Kei Islands. Reef. 22 meters. 

 Stat. 315. Anchorage East of Sailus Besar, Paternoster Islands. Up to 36 meters. 



Colony consisting of a dense clump of branches, 36 cm. in height. The branches all 

 spring from the base of the colony and almost immediately begin to subdivide. The base is 

 about 4.5 cm. broad, and the largest branches have a diameter of about 2 cm. The branches 

 divide irregularly, but ordinarily the division is not dichotomous, until ultimate branchings of 

 the 9 th or io th order are attained, the resul-t being hundreds of terminal twigs with a diameter 

 of about 3 mm. All of the branches are round and smooth, and distinctly enlarged at their 

 ends. The calyces are inserted on all sides of the branches, and are evenly spaced, averaging 

 about i mm. apart. The ccenenchyma is very thick. 



The individual calyces are completely inserted, leaving the surface of the branch perfectly 

 even, except where pitted by the calyx apertures. These pits are quite small, in alcoholic 

 specimens, the pits being oval instead of round, their greater diameter being parallel to the 



