Herbert L. Hawkins — Studies on the Echinoidea. 349 



all the features shown. By means of serial sections, it has been 

 possible to recognize that the whole girdle, in both species, is 

 practically identical with that already described. The buttresses 

 have similar characters in all three genera, but seem to be shorter 

 and steeper in Pygaster, and less massive in Holectypus. In both the 

 genera last named, the plates of the adoral surface are exceedingly 

 thin — more so proportionately than in Plesiechinus — so that actual 

 development of the inner surface is a matter of great difficulty, even 

 when the outer surface is still encased in matrix. The infilling 

 material is generally completely indurated in Holectypus, and when 

 a band of less refractory rock is found it proves to be a thin film 

 in contact with the test along a narrow belt connecting the peristome 

 with the periproct. 



4. A COMPARISON" BETWEEN THE PeRIGNATHIC GlRDLE OF PLESI- 

 ECHINUS AND THAT OF PRIMITIVE REGULAR ECHINOIDEA. 



The perignathic girdle of Plesiechinus is definitely Diademoid 

 (Centrechinoid) in character, and shows no affinity to the Cidaroid 

 type. That is to say, its major elements are situated on the 

 ambulacral plates ; the interradial ridges of the Cidaroida being 

 practically unrepresented. The interradial buttresses have evidently 

 no homology with the Cidaroid ridge, being behind the true girdle, 

 and having a merely mechanical function for the better support of 

 the fragile adoral surface. They reach their fullest development in 

 Discoides and the Clypeastroida. The sudden and complete trans- 

 ference of the "auricle" from the interambulacrum to the ambulacrum, 

 which accompanied the change from the Cidaroida to the Diademoida, 

 is one of the chief gaps still unbridged in the evolutional history 

 of the Echinoidea. The process was absolutely completed in the 

 Pygasteridae. 



The two vertical, unarched processes on the ambulacrum are in 

 keeping with the general Jurassic character for the girdle. The 

 Calycina, Hemicidaris, Pseudodiadema, Stotnechinus, and even the 

 Cretaceous Cyphosoma all possess "disjunct" auricles. And in all 

 the genera except the last named the interradial portions of the 

 girdle are very feebly developed. These features are morpho- 

 genetically neanic, or perhaps nepionic ; the girdles of later 

 Echinoids which possess elaborate structures all passing through 

 a stage comparable with this Jurassic phase. The central knob on 

 the ridge is a feature very constantly found in those Diademoida 

 Avhich have interradial developments of the girdle (e.g. Salmacis). 



But in three important respects the Pygasterid girdle differs 

 from that of the Diademoida. The processes are situated at 

 the extreme interradial edges of the ambulacra, and are not 

 perforated by the ambulacral pores. In the Diademoida the 

 processes often spring from expanded bases which involve 

 practically the whole width of the ambulacral columns, and 

 are in consequence passed through by the podial pores. In 

 Plesiechinus the processes are inclined away from the margin of the 

 peristome, while in the Diademoida they rise directly from, and 

 almost at right angles to, the margin. And lastly, the processes of 



