Vol. 56.] XJPPEK CHALK AT COTJLSDOX. 699 



Genus Rhopalastbum, Hseekel. 



Test bilateral, with three chambered arms and a central chamber, 

 which, however, in the fossil forms is not always recognizable. 



Rhopalastbum sp. a. (PI. XXXVII, fig. 26.) 



Arms short, gradually increasing in size towards the distal ends, 

 which are obtusely triangular. Length of arms, measuring from the 

 centre of the test = *125 mm. Breadth at the narrowest part =*05 

 mm. Greatest breadth ='07 mm. 



Rhopalastbum sp. (3. (PI. XXXVII, fig. 19.) 



Arms increasing rapidly towards their distal ends, which are 

 truncate. Length of arms, measuring from the centre of the test 

 = -21 mm. Breadth at base = -05 mm. Greatest breadth = "125 mm. 



Rhopalastbum sp. y. (PI. XXXVII, fig. 23.) 



Arms somewhat arched ; two terminate in club-shaped promin- 

 ences, while the third terminates in an obtuse angle. Length of arms 

 from the centre of the test =-32 mm. Breadth at base = "055 mm. 

 Greatest breadth =-10 mm. 



Rhopalastbum sp. 1. (PI. XXXVII, fig. 18.) 



Arms similar to those in the foregoing species, apparently pro- 

 ceeding from a central disc. Length of arms ='14 mm. 

 This is probably a young form. 



Genus Tbigojvactuba, Hseckel. 



Coccodiscidaa having three chambered arms on the margin of the 

 circular or triangular disc, without a connecting patagium, and 

 furnished with spines. 



Tbigonactuba abmata, sp. nov. (PI. XXXVII, fig. 22.) 



Length of arms measured from the centre, not including the spines 

 =•15 mm. Breadth at base ='075 mm. Greatest breadth = *09 mm. 

 Length of spines =-11 mm. In a smaller specimen of the same 

 species the central chamber can be distinguished, where the outer 

 part of the shell had been corroded. 



Genus Hagiastbum, Haeckel. 



The test consists of four chambered arms extending from a common 

 centre. 



Hagiastbum sp. (PL XXXVII, fig. 27.) 



Arms arranged at right angles one to the other. Greatest length 

 of entire test = -215 mm. Breadth of two opposite arms = -06 mm. ; 

 the other two are somewhat narrower, measuring -05 mm. 



There are indications of terminal spines. 



