452 MR. HERBERT L. HA^YKmS ON 



type of Pygaster Agass. (6), the name PlesiecMnus cannot be 

 retained. I have, therefore, renamed Pomel's subgenus Pygaster 

 sens. str. The ^'■Pygaster" of Pomel thus requires a new sub- 

 generic name. As this group, which is typified by P. umbrella., 

 is similar in the character of the periproct to Macropygus, and 

 seems to mark a parallel though distinct line of evolution to that 

 subgenus, I have named it Megapygus. 



Pygastbides Loven. 



As Avas realised from the first, this small recent form possesses 

 all the essential features of a Pygaster, and the species 

 {P. relictus) was originally given that generic name by Loven. 

 There is no direct evidence, so far as I have been able to gather 

 it whereby this genus should be omitted from the Pygasteridse. 

 But there is a serious doubt as to its being a " genus," in the 

 strict sense of the word. It is founded on one broken and 

 minute specimen. On first principles, the great lapse of time, 

 un bridged by any similar forms, which separates P. reUctMS from 

 even the latest members of the Holectypoida, renders it im- 

 probable that ifc can be a revived example of the group. 

 Moreover, there has recently come to light some indirect 

 evidence which seems thoroughly to undermine the foundations 

 of the " genus." The presence of a generally Pygaster-Mke, 

 facies, and of a complete lantern, in a small recent Echinoid 

 known to belong to the genus Echinoneus (Agassiz, 58), in 

 addition to the extraordinary intei'est of its mere existence, 

 makes it practically certain, to my mind, that the specimen 

 described by Loven was a similai'ly atavistic post-larval form. 

 For this reason, I have thought it best to ignore Pygastrides in 

 the diagnosis of the Holectypoida, and to omit it altogether from 

 the classification. 



Galekopygus Cotteau (Desor). 



Several well-marked features render it impossible, as well as 

 unnatural, to associate this genus with the Pygasteridfe. Two 

 striking diff"erences are the strong curvature of the two posterior 

 ambulacra at their adapical extremities (and the extreme narrow- 

 ness of the areas generally), and the irregulai'ly multituberculate 

 character of the interambulacra. Moreover, the peristome is 

 small, unnotched for branchite, and placed anteriorly from the 

 centre. It must be regarded as the earliest known genus of the 

 Nucleolitidfe of Gregory (50), and its affinities will be more fully 

 discussed in Section YI. of the present paper. 



EcHiNiTES Duncan [Protocyamus G]"egory). 



Bather has shown (55) that this " genus " (renamed by Gregory 

 in 1900), being founded on Discoidea suhacida Leske, must be 

 considered a simple synonym of Discoidea. D. subtcada is the 

 type of the genus. 



