178 MR. H. L. HAWKINS ON THE 



for the Holectypoicla, Pygaster). The fact that the compression 

 first sliows itself near the peristome is significant ; for, to my 

 mind, it strongly suppoi'ts the view that it is to the trans- 

 ference of the chief perignathic structures from the inter- 

 ambulacra to the ambulacra that the structural differences 

 between Cidaroids and Centrechinoids are to be traced. Gradually, 

 both in ontogeny and phylogeny, the region of compression 

 extends further from the point of its origin, until in many forms 

 (e. g., Echinus) it reaches practically to the ocular margin. 



From the opposite standpoint, it is true that in the Cidaroida 

 alone the ambulacra support mere granules, while every stage of 

 complexity among the Centrechinoida corresponds with a pro- 

 gressive development of large tubercles. And indeed, there is 

 no direct evidence for either side to be derived from a study of 

 Regular Echinoids, so closely do " plate-crushing " and tubercle- 

 growth coincide in their appearance. 



A study of the ambulacral structures of the Irregular Echinoids 

 has, however, convinced me that the "growth-pressure" of new 

 plates is, for them at least, the essential and original cause of 

 " plate-crushing."' 



Among the Holectypoida compound plates (on the Centr- 

 echinoid pattern) ai'e developed to an ever-increasing degree as 

 the group is traced from the Lias to the Chalk. With few excep- 

 tions, the amount of resorption of old plates at the peristome seems 

 to decrease in corresponding order; and finally, the size, both 

 relative and actual, of the tubercles steadily diminishes. In 

 Pygaster, to take a typical genus, an interesting feature can be 

 noticed in this connection. On the adoral surface the tubercles 

 become large, and their areolae often cross the sutures between 

 the ambulacral plates. The very slight degree in which the 

 development of the tubercles aifects tlie arrangement of the 

 plates can be estimated by an examination of text-fig. 41. The 

 triple arrangement of the ambulacral plating of Pygaster, and 

 presumably of the other Holectypoida, must then owe its 

 development to some more fundamental cause than that @f 

 tubercle-growth . 



In many of the larger Clypeastroida (e. g., Clypeaster), the 

 petaloid parts of the ambulacra are buiit up of alternate 

 primaries and demi-plates. The tuberculation differs in no 

 respect from that of the extra-petaloid regions of the test. Here 

 the closing round of the end of the petal affords a new point of 

 resistance to the downward passage of plates, and, as a result, 

 a secondary type of "plate-crushing" is developed. 



In the JSTucleolitidse and " Cassidulidse," although most of the 

 ambulacrum is built up of primaries, there is, near the peristome, 

 a phyllode or " hypophyllode," composed of demi- or occluded 

 plates. There is no definite difference in the coarseness or 

 arrangement of the tubercles in this region, and the structure is, 

 as I have previously shown (Geol. Mag. 1911), cognate with that 

 of the Holectypoida or Centrechinoida. 



