288 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
B. Grade Monaxonellida. 
Sub-Order 1. ASTROMONAXONELLIDA. 
2, SIGMATOMONAXONELLIDA. 
99 
C. Grade Lithistida. 
With the Lithistida I do not propose to deal, as there are no species in the 
collection. Of the other sub-orders, the Homosclerophora do not have their 
spicules differentiated into mega- and microscleres, the Astrophora and 
Astromonaxonellida both have microscleres derivable from the aster, and the 
Sigmatophora and Sigmatomonaxonellida both have microscleres derived 
from the sigma as the original type. Also it is considered that the Astro- 
monaxonellida are derived from the Astrophora, and the Sigmatomonaxo- 
nellida from the Sigmatophora. 
In this article I propose to follow the modification of this proposed by 
Hentschel (15), who abandons the “Grades” TeTRACTINELLIDA and Monaxo- 
NELLIDA completely, and divides the whole of the TreTRaxonrDa (except the 
Liraristrpa, which he does not deal with) into 3 sub-orders as follows :— 
Order TETRAXONIDA. 
Sub-Order 1. HomoscLERoPHoRa. 
2, ASTROTETRAXONIDA (= Astrophora+ Astromonaxonellida). 
3. SIGMATOTETRAXONIDA (=Sigmatophora-+ Sigmatomonaxonellida). 
Pp) 
bP) 
The second and third of these thus form homogeneous and monophyletic 
groups each immediately derived from the Homosclerophora, thus :— 
Astrotetraxonida. Sigmatotetraxonida. 
ie oa, Vi aie 
aia peneida. 
The complete list of species described in this report is appended. 
Order MYXOSPONGIDA. 
Fam. HALISARCIDAL. 
Halsarca dujardini, Johnston. 
Halisarca sp. 
Order TETRAXONIDA. 
Sub-Order Homosclerophora. 
Fam. PLAKINIDA. 
Placortis stmplex, Schulze. 
