REPORT ON THE RADIOLARIA. 553 



thin disk it nevertheless reaches unusual dimensions, its diameter in some species being 

 more than a millimetre. 



Subgenus 1. Myelastrella, Haeckel. 



Definition. — Posterior arms simple, undivided ; anterior arms lobated or cleft, with 

 one or more incisions at the distal end. 



1. Myelastrum medullare, n. sp. (PI. 47, fig. 13). 



Anterior arms liifiil, nearly square, with a shallow incision at their broad truncated end. 

 Posterior arms somewhat smaller, nearly triangular, with simple blunt ends. Sagittal constriction 

 three-fourths as large as the transverse one. Surface smooth. 



DimejisioTis. — Radius of the anterior arms 0-36, of the posterior 0'3; longitudinal constriction 0-24, 

 transverse 0'36. 



Habitat. — Pacific, central area. Station 271, surface. 



2. Myelastrum sjjinale, n. sp. 



Anterior arms bifid, twice as long as broad, with a deep incision at their truncated end. 

 Posterior arms slender, half as large, with simple blunt ends. Sagittal constriction one and a half 

 times as large as the transverse. Surface spiny. 



Dimensions. — Radius of the anterior arms 04, of the posterior 0'2 ; longitudinal constriction 0"3, 

 transverse 0'2. 



Habitat. — North Pacific, Station 248, surface. 



3. Myelastrum heteropterum, n. sp. (PI 47, fig. 8). 



Anterior arms trifid, aljout as long as broad, with two incisions at their broad trimcated end. 

 Posterior arms about half as large, simple, with rounded blunt ends. Sagittal constriction two-tlurds 

 as large as the transverse. Surface Ijristly. 



Dimensions. — Radius of the anterior arms 0'65, of the posterior 0'035 ; longitudinal constriction 

 0'4, transverse 0'6. 



Habitat. — South Pacific, Station 291, surface. 



Subgenus 2. Myelastromma, Haeckel. 

 Definition. — All four arms (posterior as well as anterior) lobated or cleft. 



4. Myelastrum octocorne, n. sp. (PI. 47, fig. 12). 



All four arms with a deep incision in their distal half ; anterior arms broader, but shorter than 

 the posterior; in the anterior arms the two lobes are of the same size, in the posterior arms the 



(ZOOL. CHALL. EXP. — PART XL.- — 1885.) Rr 70 



