738 PROCEEDINGS OF THE AMERICAN ACADEMY. 



genus, Pseudoplexaara. The new genus is characterized by them as 

 follows: "axis horny, with a central calcareous portion, the outer 

 layer of coenenchyme is soft and when dry friable; the inner layer 

 contains a number of light purple or violet coloured iiTegularly stellate 

 spicules or spindles with few rays." It is to be distinguished from 

 Plexaura, in addition, by the following features, among others: colony- 

 feebly branched, older portions of horny axis solid, younger portions 

 with calcareous particles in the center; polyps placed close together 

 in an irregular spiral, completely retractile tentacles without spicules 

 or having a circlet of them at their base; spicules mostly spiny spin- 

 dles, with numerous pink stellate forms and a few club-shaped with 

 attenuated foliaceous expansions. 



The important characters of the colony are: the relative smallness 

 of the spicules; spicules in the outer cortex, and irregularly stellate 

 forms in the inner cortex; the massing of the latter to such an extent 

 as to make the inner cortex firmer when dried, while the outer is 

 friable; the absence of spicules in the tentacles and polyps; the 

 sluggish but complete retraction of the polyps within the cortex ; and 

 the smooth cortex surface without projecting calyces in the contracted 

 or dried colony. The polyps are numerous. When they are com- 

 pletely expanded the tentacles of adjacent polyps overlap, and the 

 coenenchyme is hidden. Each tentacle has ten to twelve pairs of 

 pinnae. 



Of the three groups of alcyonarian corals, — Alcyonacea, Pennatu- 

 lacea and Gorgonacea, — only representatives of the first and second 

 have had their minute structure studied recently; the Gorgonacea, 

 to which Pseudoplexaura belongs, have received little attention except 

 from von Koch ('87) in his very important but early comparative 

 study. Studies on the Alcyonacea have been relatively numerous. 

 Von Koch ('82*) described briefly the structure of Clavularia and 

 other alcyonacean forms. Bourne ('95) described Heliopora coerulea 

 and later made a very complete study of the origin and structure of 

 its skeleton ('99). Ashworth ('99) studied the minute structure of 

 Xenia Hicksonii Ash. and Heteroxenia elizabethae Koll. He found 

 gland cells in the stomodaeum and correlated their presence there 

 with the absence of the ventral and lateral mesenterial filaments. 

 Plickson ('95) has given a detailed account of the cell structure of 

 Alcyonium digitatum, and Pratt (:05) has described the digesting 

 and mesogloea cells in several members of the Alcyonidae. She 

 found a relatively large numl)(;r of gramdar gland cells in the stomo- 

 daeum of feeding colonies and very few or none in starved ones. She 



