488 J. W. Gregory — Australian Echinoidea. 



Fam. Spatangid^;. 



Gen. Hemiaster, Desor, 1857. 



Hemiaster planedeclivis, sp. nov. PI. XIV. Figs. 6 and 7. 



Pentagonal ; thick with tumid sides : the abactinal surface is 

 depressed and flat, and slopes gently forward from the vertex : the 

 posterior side is steep, almost vertical. 



Apical area excentric posteriorly : in front of the vertex. Con- 

 struction not fully known : it is ethmolysian, and there are two 

 large pores in the antero- lateral basals. The postero-lateral basals 

 apparently also bear a pore each. 



Ambulacra : the anterior is long and narrow with small pores : 

 it is lodged in a slight depression. The petaloid portions of the 

 lateral pairs are in broader and shorter depressions. The anterior 

 pair are slightly sinuous and nearly twice as long as the posterior. 



Fasciole : peripetalous : it is irregularly hexagonal : the anterior 

 side broadens considerably in the middle. The fasciole is always 

 wider where it traverses an ambulacrum. 



Anus : high on the posterior margin. 



Mouth : at a moderate distance from the anterior margin : the 

 labium is strongly projecting. 



Dimensions. 

 Length 



"Width : at one -third length from anterior end 

 ,, ,, ,, posterior end 



Height 



Length of anterior petal 



Width of „ „ 



Length of posterior petal 



Width of ,, „ 



Apical disc : distance from anterior end 



Distribution. — Middle Murravian, Morgan, South Australia 

 (preserved in the Ipswich Museum). 



Remarks. — Hemiaster planedeclivis is a species of a very Cretaceous 

 aspect, and its closest affinity is with the group of which H.fourneli, 

 Desh., 1 H. nucleus, Des., 2 and H. palpebratus, Lor., 3 are typical 

 representatives : it has the high posterior vertex, the tumid sides, 

 the flat abactinal surface, the almost vertical posterior margin, the 

 small posterior petals, and well-developed labrum which is charac- 

 teristic of this group. Nevertheless it differs clearly from any 

 described species ; thus from H. palpebratus it can be readily dis- 

 tinguished by the greater flatness of its abactinal surface, and the 

 absence of the tumid antero-lateral interradii seen in that species. 

 The widely-distributed H. fourneli is probably its nearest ally, and 

 from this it may readily be separated by its greater breadth and the 

 flatness of the upper side. From the whole of this group of Hemi- 

 asters, in fact, the flatness of the abactinal slope, the proportions of 

 the pairs of petals, and the irregularity of the fasciole form a combina- 

 tion of characters that enable this species to be easily distinguished. 



1 Agassiz and Desor, Catalogue raisonne, pt. 3, Ann. Sci. nat. (3) viii. 1847, p. 16. 



2 Agassiz and Desor, op. cit. p. 17. 



3 De Loriol, Faune cretacique du Portugal, Echinodermes, II. fasc. 2, Lisbon, 

 1888, pp. 103-4, pi. xx. figs. 1-3. 



