Genera PseudojDjgaulus, Traclijaster, and Ditremaster. 331 



species is good for nothing, and there is no type species to 

 this one. 2. That the species mentioned as types do not 

 present the generic characters of Trachyaster. Hemiaster nux 

 has not four genital pores with the posterior basals separated 

 by the madreporite ; it has only two. It belongs, according 

 to the method of M. Poniel, to the genus Ojjissaster, Porael 

 {op. cit. p. 37), and its synonym, Ditremaster^ Munier-Chal- 

 mas, of which we shall write presently. Hemiaster digonus 

 is well known to us, as it is a common species in Sind, and it 

 has not four genital pores, so as to be a Trachyaster. The 

 madreporite, moreover, does not always project between the 

 ocular plates. Extraordinary as are these mistakes, those 

 which follow are still more so, and simple want of observa- 

 tion will hardly explain the assertion that the madreporite is 

 probably prolonged between tlie posterior ocular plates in H. 

 gihbosus and H. zonatus. (M. Pomel forgets to place the 

 name of A. Agassiz after these species.) 



It is a positive fact that in Hemiaster gihhosus, A. Agassiz, 

 the madreporite is restricted to the right anterior genital 

 plate, and that it in no way passes between the posterior 

 genital plates. It is a perfectly Ethmophract Hemiaster. 

 The drawing of the apical system in the Report on the '• Chal- 

 lenger' Echini, pi. xx. a fig. 11, sets this matter beyond 

 dispute. Hemiaster zonatus^ A. Agassiz, is also drawn upon 

 plate XX. a of the ' Challenger ' Report, and there is abso- 

 lutely no warrant for M. Pomel's doubt as to the nature of the 

 apical system ; the specimens are figured covered with their 

 spines, and it is only the distinguished naturalist who has 

 remarked upon the species that is in a position to know any- 

 thing about it. But A. Agassiz remarks that the species 

 only differs from H. expergitus^ Loven, in characters which 

 are to be referred to age, and Loven's species has most defi- 

 nitely the madreporite restricted as in H. gihhosus. A. 

 Agassiz compares H. zonatus with H. gihhosus^ and says 

 nothing about an unusual extension of the madreporite. 



It is indeed to be regretted that M. Pomel did not study the 

 variations in the numbers of genital pores and the variable 

 extension of the madreporite in individuals of some common 

 recent species of Echinoidea. Had he done this he would 

 have seen that no satisfactory generic characters are to be 

 obtained from the number of pores and the size of the madre- 

 porite, all other structural characters being the same. 



Neither M. Cotteau, M. Pomel, nor M. Munier-Chalmas, 

 whose work we have to criticize shortly, appear to have studied 

 the admirable work of Loven, in his ' Etudes ' and in his 

 * Pourtalesia ' (Kongl. Svenska Vet.-Akad. Handl. Bd. x. 



