1892.] MIcnoSCOPlCAL JOURNAL. 217 



iya^ Hy7neniastrum ^ or one of its two allies. If there are no 

 spines or chambered arms it is either the first or second, and an 

 equatorial girdle will make it Pcrichlamydiuni. Stylodictya 

 has ^ or more spines. IIv»icniastr?t7ii^ Histiastrum and Stcph- 

 atiastrum have radial chambered ar9?is with or without solid 

 spines at the ends ; the first has 3 arms, the second and third 4. 

 Stephana strum may be known by the presence of a terminal 

 girdle. 



The student will note that some of these genera are very much 

 alike. Hyvienactura and Ilymcmastrjim both have 3 radial 

 chambered arms and connecting membrane or network between 

 the arms, Staiiractzira and Histiastriini both have 4 such arms 

 and connecting membrane. So that it is hard to distinguish 

 these genera respectively. The point to note especially is whether 

 the surface is convex, /. e., lens-shaped, or comparatively flat. 

 If the former, it is Hyvienactiira or Stauractiii-a^ as the case 

 may be ; if the latter, Hynieiiiasti-iim or Histiastrutn. The 

 presence of the porous sieve plate is also another mark by which to 

 recognize Hvmoiiastrujii aad Histiastriini. 



Passing now to the second group, which is as a rule character- 

 ized by the presence of one large mouth or opening instead of 

 many small ones, we have these 36 genera to consider : 

 Lithocircus^ DictyocepJiahis^ 



Zvgocircus^ Pterocaniuni^ 



Zvgostephanus^ Pterocodon^ 



Acanthodesmia ^ Podocyrtis^ 



Se7nantis= Stephanolithis^ Thvrsocyrtis^ 



HaJicalyp t ra , Dictyopodiii m , 



Cor7iutella\, Lit lior7tith 171771^ 



Di.ctyophi77ius^ Rhopaloca7iiii77i ^ 



Litho77ielissa^ Lithochyt7-is^ 



LychnocaTiium^ Cycladophora^ 



Lithope7-a^ Calocyclas^ 



Setha77ipho7'a (= C7-yptop7'07-a Ii .), Eucy7'tidi7i777^ 

 Anthocyrto77ia^ Lithoca7iipc^ 



A7tthocy7-tis^ Petalospy7-is^ 



A7ithocyrtiu77i , Ce7-atospyris^ 



Ant/iocy7'tidiu77i, Cladospy7'is^ 



Ca)'poca7tiu77i^ Lithobotrys^ 



Lophophce7to., Potryocyrtis, 



A formidable array certainly but not so hard to master as 

 would appear at first sight, since they admit of several subdivi- 

 sions very plainly marked. Thus in the first five of the above 

 genera the skeleton is 



A Ring or Combination of Rings. 



In Lithoci7-cus and Zvgocircus there is but a single ring and 

 branched spines distinguish the former. 



