Herbert L. Hawkins — Studies on the Echinoidea. 343 



high relief. Specimens of Pygaster semisulcatus (olim " P. umbrella") 

 are frequently found in the Corallian of Berkshire in the state of 

 decorticated moulds, and in such examples the presence of strong 

 perignathic prominences is indicated by deep pits on the adradial 

 sides of the branchial incisions. But attempts to reproduce these 

 perignathic structures by gutta-percha casts have hitherto proved 

 unsuccessful. The pits are so deep that it is almost impossible to 

 ensure their perfect emptiness ; and they are internally expanded 

 and curved. For these reasons our knowledge of the perignathic 

 girdle of the Jurassic Holectypoida has up to the present been 

 practically confined to a realization that such a structure existed in 

 robust development. Loven (" Recent Form of the Echinoconidse " 

 and " Echinologica") ascertained something of its character by 

 means of sections, but was able to examine the more prominent 

 features only. 



After many failures, I have succeeded in exposing the girdle, with 

 varying completeness, in two fully grown specimens of Plesiechinus 

 ornatus (olim Pygaster semisulcatus) from the Aalenian (Pea Grit) of 

 Leckhampton. In many examples there is a cylinder of fairly 

 yielding matrix connecting the apertures of the periproct and 

 peristome, while the limestone wholly sheltered by the test is in 

 a very refractory state. In the two specimens on which this paper 

 is based, the soft plug, though containing many pisolite grains, was 

 slightly greater in diameter than the peristome ; and so it was 

 possible, by slicing off the adapical surface, to expose all of the 

 perignathic structures in one instance and the adoral parts of them 

 in the other. The more completely developed specimen is now in 

 the collection of the Geological Department, University College, 

 Reading, registered number 924. 



The structures shown in the prepared specimens have a special 

 interest in view of the primitive qualities of the genus Plesiechinus. 

 In the present paper a brief comparison is made between the 

 perignathic girdle of the Pygasteridse and that of the simpler types 

 of the Diademoida (Centrechinoida). For the purpose of this 

 comparison I have used Jackson's summary and diagrams given in 

 his " Phylogeny of the Echini ", pp. 189-195, reinforced by a few 

 observations of my own. A detailed comparison with the later 

 Holectypoida and the Clypeastroida (in which, thanks to the work 

 of Loveu and Duncan, the girdle is already well known) is postponed 

 to a later paper partly prepared. 



2. The Perignathic Gikdle of Plesiechinus ornatus. 

 (a) The relations of the basi-coronal plates . (See p. 344, Fig. 1.) 

 The peristome of Plesiechinus (and of all Jurassic Pygasteridse) 

 is approximately circular in outline, but the circumference is 

 strongly " festooned " by profound branchial incisions. These 

 incisions, as is usual in Ectobranchiate Echinoidea, involve those 

 parts of the interambulacral margin that are contiguous to the 

 ambulacra. As may be seen by reference to Fig. 1, the branchial 

 incisions are not equidistant, the measurement across an ambulacrum 

 being greater than that across an interambulacrum. The actual 



