Dr. F. A. Bather — Eocene Echinoids from Sokoto. 297 



surface is gently domed, the petals narrow and very slightlj'^ con- 

 vergent ; P. placenta is the nearest to our species and particularly 

 resembles B, but, as compared with the unworn A, it is much thinner 

 and flatter, while its ornament is said to be larger and scantier on 

 the actinal surface, the reverse of P. Saharce. 



Evidence as to age. — All species of this genus pi'eviously 

 described are found in Sind. P. elongata comes from the Strata 

 below the Trap, which appear to be at the base of the Tertiary. 

 The other species, some of which are more like P. Saharce, come 

 from the top of the Eanikot Series, which is thought to be Lower 

 Eocene. There are in P. Saharce no signs of more advanced 

 evolution such as might suggest for it a later date. 



Hemiaster Desor, 1847. 



Ann. Sci. Nat. (3), viii, p. 16. 



SjTi. Trachyaster Pomel, 1883, Genera des Ech., p. 38. 



There has been much discussion and uncertainty about the limits 

 of this genus, and a summary of the position in the year 1889 is 

 given by Duncan (Revision of Echinoidea, p. 225). However 

 the genus be dismembered, it should retain as genotype the species 

 described by A. Brongniart (1822) as Spatangus hnfo, which was 

 the first in Desor's list, and was subsequently referred to by him as 

 characteristic of the genus. For present purposes it is unnecessary 

 ^to consider whether or not Ahatus, Tripylus, Ditremaster, and many 

 ■other forms at one time or another placed with Hemiaster, should be 

 left in the genus. The only one with which the specimens before 

 us have anything to do is Trachyaster. In the genotype and other 

 characteristic species ol Hemiaster, the apical system is compact and 

 ethmophract (the madreporite bounded posteriorly by genitals 1 

 and 4), and the compact nature of the system was recognised in 

 Desor's revised diagnosis (Synopsis, 1858, p. 367). In certain 

 species that would otherwise fall into Hemiaster, the apical system 

 is ethmolysian (the madreporite extending backwards between 

 jicnitals 1 and 4 and oculars I and V). For such species the genus 

 Trachyaster, proposed by Pomel, was adopted by Cotteau (1887, 

 Paleont. frang., Terrains Tert., Echinides Eocenes, p. 400). Duncan 

 and Sladen (Ann. Mag. Nat. Hist. [6], ii, p. 329; 1888) rejected 

 this genus for reasons partly bibliographic, partly morphological. 

 Their objections of the former class were, however, based on an 

 incomplete acquaintance with Pomel's writings, many of which 

 were not accessible in this country till the death of their author. 

 They stated, in fact, that the genus had been proposed without any 

 genotype ; and it is true that in the section of his work first pub- 

 lished (1883) Pomel merely said " Le type est fossile du miocene 

 ■superieur." But from the 2eLivraison, p. 108 (1888), it is clear that 

 the species intended was Trachyaster globulus Pomel, which was 

 figured as T. globosus in the plates, first issued in 1887. There can 

 therefore be no technical objection to the acceptance of Pomel's 

 name. If there were, one would merely fall back on Cotteau's 

 diagnosis and the genotype T. Heberti Cott. But Duncan & Sladen 



