H. L. Hawkins—Amobulacral Structures in the Holectypoida. 351 
and so material for a genetic study of the forms merely awaits the 
accumulation of sufficient zonally-collected specimens. 
Pygaster, Agass., first appears in the Upper Lias, and is possibly 
represented in the Middle Cretaceous by P. truncatus, Agass. The 
genus is, however, practically confined to the Jurassic Period. 
Holectypus, Desor, originates in the Inferior Oolite, and continues 
into the lower part of the Upper Chalk (Z. serzalis). 
Discoidea, Gray, is an essentially Cretaceous genus, commencing in 
the Lower Gault and disappearing at about the same horizon as does 
Holectypus. 
PYGASTER HOLECTYPUS DISCOIDEA CONULUS 
SIACROCYPHUS SERIALLS FORGEMOLLI ALBO-GALERUS 
Fie. 2. Comparison of the plate-crushing in four Holectypoid genera. 
Cenulus, Leske, is another purely Cretaceous genus, first met with 
in the Upper Gault and persisting to the top of the Upper Chalk. 
Considered in this order (that of their appearance in time), these 
four genera exhibit a fairly uniform series. Pygaster, with its 
periproct but slightly removed from the apex, and its well-developed 
jaws and external branchie, shows marked affinities with the Regulares; 
and Conulus, with a marginal periproct.and probably no true jaws 
(when adult), is the most remote from the regular condition. 
