Herbert L. Hawkins— Studies on the Echinoidea. 391 



of their diagram given by the last-named authors affords a most 

 striking contrast with that which must inevitably apply to Loven's 

 drawing. They maintained that the straight suture which comes at 

 the adradial base of the projecting part of the girdle was the 

 " interradio-ambulacral " (adradial) suture, and that the three large 

 constituents of the girdle were all wholly interradial in position. 



Two such opposite interpretations of a superficially straight- 

 forward structure demand reconciliation. The chief purpose of this 

 paper is to show how this reconciliation can be effected. In the 

 next paper of this series a new reading of the girdle of Conuhis will 

 be given (the denial of the presence of this structure is a lamentable 

 and inexplicable lapse on the part of Duncan & Sladen), together 

 with a general summary of the characters of the perignathic girdle 

 in the Order. I hope to publish a comparison of the girdles of the 

 Holectypoida and Clypeastroida at an early date, at the same time 

 discussing the homologies of the peristomial structures of the 

 Spatangoida. 



2. The Perignathic Girdle of Discoides cylindhicus. 

 (a) Duncan 8f Sladen'' s Interpretation. (PI. XXV, Fig. 1.) 



The outstanding characters of the perignathic girdle as represented 

 by this diagram are : firstly, the presence of a low, thickened margin 

 to the peristome in the ambulacral regions, perforated by three pore- 

 pairs in each area; and secondly, the development of smooth, 

 sloping ridges on the interambulacral margins, each composed of 

 three flat plates, of which the central one is large and roughly 

 rectangular in shape, while the outer (adradial) two are narrow 

 adorally, expanding distally into prominent "ears". There is 

 nothing inherently improbable in the ambulacral part of the girdle, 

 for it would prove to be in a slightly modified Cidaroid stage of 

 development. But the existence of three plates, representing three 

 columns, in the interambulacra, would be a most extraordinary 

 anomaly. It is true that in the somewhat obscure Tiarechinus and 

 Lysechinus from the St. Cassian Beds of the Trias, the interambulacra 

 are believed to show three columns (represented by one plate each) 

 above the primordial, but to find such a feature in Discoides, especially 

 when the rest of the interambulacra are built of the normal two 

 columns, would be indeed marvellous. Even if the large central 

 plate of the ridge be regarded as the primordial, and the two plates 

 at the sides as the first two paired plates of the area, the figure 

 becomes only less grotesque. 



A comparison of the figure with a prepared specimen shows that 

 the former is, as regards its drawing, a fair, though diagrammatic, 

 representation of the appearance of the girdle. But the sutures near 

 the ambulacral pores are not so straight as Duncan & Sladen make 

 them, and the lateral "eared" plates of their ridge are certainly 

 based upon the proximal ambulacral plates. There can be no 

 possible doubt that their " interambulacro-radial " suture is 

 really the basal suture of the process. The two "eared" 

 lateral plates of the ridge are therefore certainly processes, as 

 Loveu maintained. The true ridge, stripped of its radial 



