KEPORT ON THE RADIOLAEIA. 539 



Subgenus 1. Trigonastrella, Haeckel. 



Definition. — All three arms of the same size and form, equidistant; fundamental 

 form of the shell therefore a regular, equilateral triangle. 



1. Trigonastrum regulare, n. sp. (PL 43, fig. 16). 



Chitonastrum regulare, Haeckel, 1881, Prodromus et Atlas (pi. xliii. fig. 16). 



All three arms equal and equidistant. Each arm has the form of an isosceles 'triangle, twice 

 as high as broad, the truncated apex of which is inserted into the large circular central disk, 

 whilst its distal base (four times as broad) is divided by a deep incision (half as long as the arm)- 

 Each arm with ten to twelve joints. Patagimn between the arms nearly complete, spongy- 

 (Differs from Uhitoiiastrum triglochin mainly in the possession of a patagium.) 



Binunsions. — Eadius of each arm 0"24, greatest breadth Oil, basal breadth 0'03 ; length of the 

 aides of the regular triangle 0'45. 



Hahitat. — Pacific, central area. Station 274, surface. 



Subgenus 2. Trigoiiastromma, Haeckel. 



Definition.^One odd arm different in form or size from the other two arms, which 

 are paired ; distance between them difierent ; fundamental form of the shell therefore 

 an isosceles triangle. 



2. Trigonastrtim krohnii, Haeckel. 



Euchitonia krohnii, Haeckel, 1862, Monogr. d. Eadiol., p. 507. 



AE three arms different; distance between the two paired arms smaller than their distance from 

 the odd arm.. Odd arm with six joints. Each paired arm With seven joints, increasing in breadth 

 towards the distal end. Odd arm and one paii'ed arm forked at the end, the other paired arm 

 simple. Patagium nearly complete. (The asymmetry in this form may perhaps be an individual 

 anomaly, as also in Mydastrum anomalum., PI. 47, fig. 9.) 



Dimensions. — Radius of the arms about 0*13 to 0'16, breadth 0"05 to 008. 



Habitat. — North Atlantic, Funchal, Madeira, Krohn, surface. 



3. Trigonastrum gegenbauri, Haeckel. 



Euchitonia gegenbauri, Haeckel, 1862, Monogr. d. Eadiol., p. 506, Taf. xxxL figs. 2, 3. 



Arms different; distance between the paired arms larger than their distance from the odd arm, 

 which is one-third shorter. Odd arm egg-shaped, simple, with seven joints, undivided. Both paired 

 arms equal, with ten joints, in the distal third forked. Patagium nearly complete. 



Dimensions.— Badiws of the odd aim 014, greatest breadth O'OS ; radius of the paired arms 0'2. 



Habitat. — Mediterranean (Messina), Haeckel, surface. 



