530 THE VOYAGE OF H.M.S. CHALLENGER. 



expressed in the diagnosis given above, and call the former state (the Coccodiscid) 

 Hymenactura (compare above, p. 473). One practical advantage, obtained in this, way, 

 is that all genera of Discoidea ending with "-astrum " belong to the Porodiscida. 

 Hymeniastrum differs from Dictyastrum by the possession of a patagium, and from 

 Euchitonia by the equal size of the angles and the arms. 



Subgenus 1. Hymenastrella, Haeckel. 

 Definition. Arms with blunt ends, without terminal spines. 



1. Hymeniastrum leydigii, Haeckel. 



Euchitonia leydigii, Haeckel, 1862, Monogr. d. Eadiol., p. 510, Taf. xxxi. figs. 4, 5. 



Hymeniastrum leydigii, Haeckel, 1881, Prodromus, p. 460. 



Histiastmm trinacrium, Haeckel, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 843. 



Arms twice as long as broad, two-thirds as broad at the base as at the blunt, nearly truncated 

 distal end. Patagium nearly complete, perfectly filling out the interbrachial spaces, with six to 

 seven regular, concave chamber-rows; only the truncate terminal faces of the arms free. 



Dimensions. Eadius of the arms O15 to 0'18, breadth on their base O04 to O'Oo, on their 

 broadest distal end 0'06 to 0'07. 



Habitat. 'Cosmopolitan ; Mediterranean, Atlantic. Pacific, surface. Very common and 

 variable. 



2. Hymeniastrum Jcollikeri, Haeckel. 



Euchitonia fcollikeri, Haeckel, 1862, Monogr. d. EadioL, p. 511, Taf. xxxi. figs. 6, 7. 

 Hymeniastrum Jcollikeri, Haeckel, 1881, Prodromus, p. 460. 



Arms nearly lanceolate, twice as long as broad, half as broad at the base as at the broadest 

 middle part, pointed at the distal end. Patagium complete, enveloping the whole triangular disk, 

 with irregular chamber-rows. 



Dimensions. Eadius of the arms 0'2, breadth on their base 0'04, on the broadest part 0'08. 



Habitat. Mediterranean (Messina), surface. 



3. Hymeniastrum gumbelii, Haeckel. 



Stylaetis gumbelii, Stohr, 1880, Paleeontogr., vol. xxvi. p. 112, Taf. vi. fig. 1. 



Arms twice as long as broad, nearly twice as broad in the circular distal half as in the square 

 proximal half. Patagium incomplete, circular, with eight to nine convex chamber-rows, enveloping 

 only the proximal square halves of the arms ; the lenticular distal halves remain free. 



Dimensions. Eadius of the arms 0'14 ; breadth at the base 0'04, at the distal lenticular 

 part 0-07. 



Habitat. Fossil in the Tertiary rocks of Sicily, Grotte, Stohr. 



