G-ENERA A]N"D GROUPS OF THE ECHINOIDEA. 229 



p. 1076), quoted by M. Cotteau in Pal. rran9. (1887, Terr. 

 Eocene, p. 411), is the same thing as Abatus, sensu Loven, and ' 

 is synonymous mth Tripylns (see Duncan & Sladen, 1888, Ann. 

 & Mag. Nat. Hist. ii. p. 333). The history of the species " caver- 

 nostis" should have been studied by the distinguished French 

 Echinodermatists. Ditremaster is of no classificatory value. 

 It appears to be reasonable, under the circumstance?, to add 

 to and alter slightly the Desorian diagnosis of Hemiaster, and 

 to divide the genus into two types in reference to the position 

 of the madreporite, the number of genital pores depending 

 much upon the position of that upper end of the water-system. 

 Einally Tripyhis, with its persistent or evanescent latero-anal 

 fasciole, may remain as a subgenus which will include Ahatus, 

 and it will come within the second division of the genus. I have 

 found that Mr. Bone, who was Dr. "Wright's artisb, was correct 

 in drawing a peripetalous fasciole in Epiaster Be Loriolii, 

 Wriglit, The species was placed in Hemiaster by Dr. S. P. 

 Woodward ; and therefore there is a large Micraster-like Hemi- 

 aster in the Upper G-reensand with a delicate peripetalous fasciole, 

 and this is the sole distinction from Epiaster. 



Grenus Hemiaster, Desor, 1847, Cat. Bais., Ann. d. Sci. Nat. 



vol. viii. p. 16 ; 1858, Synopsis, p. 367. A. Agassiz, 1872-74, 



Revision,^. 585 ; 'Sassier' EcJiini,^. 20; 1881, 'Cliallenger^ 



EcUni, p. 177, pi. xx a. E. A. Smith, 1879, Phil. Trans. 



Royal Soc. vol. clxviii. p. 271. Loven, Etudes, 1874, pis. xi. & 



xxix. ; 1883, Pourtalesia, p. 72. Gauthier, 1887, Assoc. 



Frang. Sci. p. 406. Duncan Sf Sladen, 1888, Ann. Sf Mag. 



Nat. Hist. ser. 6, vol. ii. p. 329. Be Loriol, 1888, Eaune 



Cret. du Portug., Ech. vol. ii. fasc. 2, p. 98. 



Syn. Trachyaster, Pomel ; Ahatus (sensu Loven) ; Leucaster, 



Gauthier, 1887 ; Perionaster, Gauthier, 1887 ; Ditremaster, Mu- 



nier-Chalmas, 1885 ; Opissaster, Pomel (pars). 



Test moderate and rarely large, cordiform in tumid marginal 

 outline, with a more or less well-defined anterior groove, 

 narrowest and truncated posteriorly, undulating or gibbose at 

 the sides, short or longer tlian broad, highest subcentrally or 

 posteriorly to the true centre, varying in height, tumid in the 

 interradia dorsally, often keeled posteriorly to the vertex. 

 Actinal surface swollen behind tlie peristome. 



Apical system subcentral, with two to four basal plates, rarely 



