ECHINOIDEA 323 



Schizaster L. Agass., is represented in the Mediterranean by the 

 species canaliferus (Lamarck) — perhaps rather to be regarded 

 as the type of a separate genus, Ova Gray, and thus to be 

 designated Ova canalifera (L. Agass.) ; even though it may probably 

 be found also along the Atlantic coasts of Morocco and Spain, 

 there is no probability of its extending north to the British area. 

 It is related to Brisaster, but easily recognisable by its high, 

 vertical posterior end, two genital pores, and the crowded pores 

 of the frontal ambulacrum, arranged in double series. 



Key to the genera of Heyniasteridoe known from the British 

 {and N.E. Atlantic) seas. 



Only a peripetalous fasciole present ; test high ; frontal ambula- 

 crum narrow, only slightly deepened 



1. Heyniaster Ag. and Desor 

 A lateral fasciole issues from the peripetalous fasciole ; test low ; 

 frontal ambulacrum very broad and much deepened 



2. Brisaster Gray 



1. Hemiaster L. Agassiz and Desor. 



Test rather high, scarcely elongate. A conspicuous, nearly 

 circular peripetalous fasciole, but no other fascioles. Frontal 

 ambulacrum narrow, scarcely deeper than the lateral petals. 

 Four genital pores. The madreporite not prolonged back- 

 wards so as to separate the posterior genital and ocular plates 

 (Fig. 188, 1), the apical system being of the " ethmophract " type. 



Only one species of this genus known from the British (and 

 N.E. Atlantic) seas. 



1. Hemiaster expergitus Loven. (Fig. 186.) 



(Syn. Hemiaster zonatus A. Agass. ; H. Mentzii A. Agass. ; 

 H. gibbosus A. Agass.) 



Test nearly round, high, but flattened above, gently rising 

 towards the posterior end, which is vertical. Petaloid area small, 

 the surrounding peripetalous fasciole rather distant from the 

 ambitus. The posterior paired petals about half as long as the 

 anterior ones. The sternum very wide, the width distinctly more 

 than haK the length. Valves of globiferous pedicellariae ending 

 in a transverse- oval opening surrounded by usually 6 teeth 

 (Fig. 187, 1). Colour in life unknown, but probably jdlowish- 

 brown. Largest size recorded, 53-5 mm. length. 



