330 Messrs. Duncan and Sladen's Ohjections to the 



but the connexion of this genus with Hemiaster^ Desor, is 

 impressed upon the reader. 



Inasmuch as Trachyaster is a genus which was not foreseen 

 by Forbes, Bittner, de Loriol, and Desor, which is said to be 

 allied to Hemiaster and Periaster^ while part of it forms a 

 genus Ditremaster^ and considering that not one species of it 

 properly bears the name of M. Pomel after it, and that we are 

 not informed concerning the typical species of the genus 

 according to M. Pomel, the whole history of the genus 

 requires, in our opinion, very decided criticism. 



The genus will be found in Pomel, ' Theses presentees k 

 la faculte des Sciences de Paris,' 1883 (published at Algiers), 

 p. 38. The first thing which strikes the student of this work 

 is that Trachyaster is placed just before Abatus, Loven, and 

 Palceostoma, Lovdn, and that it follows a new genus Opiss- 

 as^er, Pomel, which has two genital pores. Preceding the genus 

 are Moira, ScJiizaster, &c., but there is no sign of Hemiaster ^ 

 Desor. After the " Brissiens," which contain these genera, 

 come the " Philobathidds," with Aceste ; then come the Pour- 

 talesiadae, and then the " Progonast^rides," and in a division 

 of these — the " Pycnastdrides '"' — we find Pericosmus, Peri- 

 aster^ a genus Mecaster, Pomel, and then Hemiaster. These 

 Progonast^rides form a subfamily differing from that in which 

 Trachyaster occurs. It is clear, then, that according to M. Po- 

 mel there is a greater classificatory gap between that genus 

 and Hemiaster than M. Cotteau supposes. On examining 

 the diagnosis of Trachyaster and on comparing it with 

 that of Mecaster, Pomel (op. cit. p. 42), their superfluity is 

 evident. 



The diagnosis is as follows : — "Trachyaster, Pomel. Globu- 

 lar, with the apex excentric behind; four pores (genital). 

 Anterior ambulacrum simple, in a shallow groove, lost in 

 front, and notching or not the test at the ambitus; petals 

 depressed, unequal, oval or oblong, the anterior sometimes 

 slightly flexuous at the summit. Peripetalous fasciole an- 

 gular ; peristome labiate, not very close to the margin. 

 Periproct at the top of the posterior part, above a more or less 

 marked depression. Tubercles close.^' 



A very important statement is then made : — " The type is 

 a fossil of the Upper Miocene [no name is given] ; it is 

 necessary to unite with this the greater part of the Tertiary 

 Hemiaster s, such as H. nux, H. digonus, H. rotundus, &c., 

 which have the madreporite prolonged between the posterior 

 ocular plates, and, probably, i/. gihbosus and H. zonatus of the 

 recent fauna." 



We remark : — I. That a genus without a described type 



