Verrill, as usual, was quick to discern the most important characters by which this 

 family can be differentiated from its allies. His definition is as follows: 



"Corallum usually dichotomous or more or less arborescent. Axis horn-like, or more or 

 less calcareous, especially at base. Longitudinal ducts equal, arranged regularly all around the 

 axis. Coenenchyma usually thick. Cells scattered over all parts of the surface, flat, or elevated 

 on prominent Verrucae. Tentacles at base, and sides of the polyps stiffened with large fusiform 

 spicula. Spicula of the ccenenchyma usually large, of various forms, most usually these are large 

 warty spindles mingled with clubs or crosses". 



For the purposes of this work the following definition is offered : 



Ccenenchyma thick, without evident grooves; axis cylinder horny, or horny and calcareous, 

 but never jointed. Primary water-vascular canals in a regular series around the axis. Calyces 

 on all sides of the stem and branches, often entirely included. Spicules various, usually spindles, 

 clubs, butterfly-shaped or Blattkeulen. 



As Wright and Studer remark: "The genera belonging to the family as it now stands, 

 certainly require revision". 



The material secured by the Siboga Expedition that can be referred to the Plexauridse 

 is not sufficient in extent to justify the author of this report in undertaking a revision of the 

 family, and he therefore contents himself with recognizing the genera defined in the Challenger 

 Report, and the addition of a single new genus which seems necessary to accommodate a 

 remarkable species collected by the Siboga Expedition. 



The most important feature in the diagnosis of genera in this family seems to be the 

 character of the axis cylinder, the forms of the calyces and spicules being however, of great 

 service in generic description in some cases. 



The following artificial key may be of service in aiding the student to determine the 

 generic affinities of species. 



Artificial key to the genera of Plex.a.urid.e. 



Stem, branches and horny axis distinctly flattened (Platygorgia) ^ 



Calyces prominent and bilabiate (Eunicea) 



Calyces moderately prominent, not bilabiate. 



Spicules armed with long, truncate Verrucae (Anthoplexaura) 



Spicules small, the outer layer club-shaped (Paraplexaura) 



Calyces neither bilabiate nor decidedly prominent. Axis not distinctly flattened. 

 Axis entirely horny, except at base, or with calcareous particles deposited 

 in the central core. 

 Spicules usually spindles, horizontally or longitudinally placed in outer 

 layer of coenenchyma. 

 Coenenchyma thick. No very large, bar-like spicules Plexaura 



' Generic names enclosed in brackets indicate that such genera are not represented in the Siboga collection. 



