168 Herbert L. Hawkins — Studies on the Echinoidea, etc. 



Some further apology for the reinstatement of a genus rejected by 

 so many authorities seems necessary. The original diagnosis given 

 by Pomel is as follows: " Ce sont des Pygaster dont l'apex est 

 forme de quatre genitales peu inegales, disposees en demi-cercle, 

 ainsi que les ocellaires posterieures, et formant le cadre superieur du 

 periprocte, qui est oblong et ties vaste." The only criticism that can 

 be made concerns the final words. The periproct in P. macrostoma is 

 certainly large, but not so extensive, in proportion, as it is in many 

 species of the true Pygaster. The reference to its shape, and the 

 description of the apical system, hold good. As will be found in the 

 detailed analyses that will appear in later papers of this series, there 

 are many other features in which the species of Plesiechinus resemble 

 one another and differ from those of the other Pygasterinse, but I feel 

 convinced that, in this case, the characters of the apical system are 

 alone sufficient to render the genus valid. . The most obvious, as 

 well as most fundamental, difference between Regular and Irregular 

 Echinoids lies in the relation between the periproct and the apical 

 system. Among the Pygasteridse we have the history of their 

 separation gradually revealed. The first stage is marked by the 

 destruction of the posterior genital plate, and the splaying out of 

 the apical system to give passage to the retreating periproct. Later 

 stages are concerned with the closing in and readjustment of the 

 plates of the system when the aperture has been sufficiently with- 

 drawn. Plesiechinus, as here defined, comprises all species that 

 show the first stage of periproct-migration ; the other Pygasteridse 

 include representatives of the later conditions. There are thus sound 

 phylogenetic reasons for the genus, which are rendered the more 

 secure when it is seen that all the species referable to it with 

 certainty are the earliest members of the family. On both morpho- 

 logical and stratigraphical evidence we find in Plesiechinus " le type 

 le plus rapproche des globiformes " (Pomel). 



Pygaster reynesi, Desor, and Clypeus ornatus, Buckman, are two 

 well-defined species of Plesiechinus, in addition to the genotype, 

 P. macrostoma, Wright. 



5. Summary. 



The following list includes all the changes rendered immediately 

 necessary as a result of the previous discussion : — 



PrGASTER, Agass., 1836 (incl. Jlfegapygus, Hawkins, 1912). 



Type, Clypeus semisulcatus, Phill. Corallian (incl. Pygaster 

 umbrella, pars, auct.) (non Pygaster semisulcatus, auct.). 



Plesiechinus, Pomel, 1883 (incl. Pygaster sens, str., Hawkins, 1912). 



Type, Pygaster macrostoma, Wright. Bathonian. 

 Generally speaking, Pygaster is a Middle and Upper Oolitic group, 

 and Plesiechinus is restricted to the Lias and Lower Oolite. 

 » 



