Herbert L. HawJcins — Studies on the Echinoidea. 11 



But tliis locally thickened region is merely a rim, and no trace of any 

 structure even remotely suggestive of a perignathic girdle can be 

 detected in it. It is not undercut adambitally, but is imperceptibly 

 reduced to the average thickness before the invagination is passed. 

 The ambulacral part of the peristome border is remarkably thin, and 

 shows absolutely no indication of any kind of specialization. It is 

 simply the inverted edge of the proximal ambulacral plates. There 

 is thus no vestige of a perignathic girdle in the observed genera of 

 this Order. 



The complete and sudden disappearance of the jaws and everything 

 pertaining to them in the Nucleolitoida is mysterious, but by no 

 means surprising. Before the advent of the Oolitic period, while yet 

 the Diademoida and Holectypoida had hardly entered upon the paths 

 of the evolution of their buccal structures, the Nucleolitoida had 

 utterly abandoned all traces of such apparatus. An ontogenetic 

 parallel is afforded by the young Ecliinoneus, in which a lantern 

 and girdle are almost completely developed, both disappearing 

 simultaneously at a later stage of growth. In the absence of any 

 development capable of correlation with the perignathic girdle, it is 

 impossible to make comparisons between the JTucleolitoida and the 

 Holectypoida. But the complete diversity of the two Orders in this 

 respect, coupled with their contemporaneous and early appearance, 

 suggests that they originated independently. Whetlier they sprang 

 from a common stock or are fundamentally distinct, is a problem 

 which cannot be attacked on these lines of argument, 



5. The Cassiduloid Peristome. 



I have examined the interior of the peristoraial region in Conulopsis 

 {Echinoconus, Desor) only among the many representatives of this 

 Order. The evidence thus obtained, though interesting and 

 suggestive, cannot therefore be considered adequate for the formula- 

 tion of any definite hypothesis. It seems, however, sufficient to 

 indicate the phyletic distinction between this Order and the 

 Nucleolitoida, with which it is generally associated. 



Externally, the adoi-al region in tliis Order is characterized by the 

 expansion of the proximal parts of the ambulacra into variously 

 developed phyllodes, separated from one another by more or less 

 prominent " bourrelets" on the interambulacra. (Many of the more 

 elaborate Nucleolitoida, such as Clypeus and Pygnrus, are similar in 

 this respect.) Internally, the thickening of the interambulacra is 

 almost like a reflectioTi of their external character, so that tlie 

 interradial margins of the peristome are excessively massive. 



In Conulopsis aihreviata, from the Upper Chalk of Norfolk, 

 the phyllodes are practically non-existent, although pronounced 

 " bourrelets" are developed. An internal view of the adoral surface 

 shows a remarkable resemblance to that seen in Comdus^ and is of 

 itself enough to render the generic name morphologically appropriate, 

 whatever may be its systematic fate. The ambulacra are terminated 

 adorally by an almost unthickened edge, and so appear as five grooves 

 radiating from the slightly elliptical aperture of the peristome. The 

 interambulacra increase steadilv in thickness from the ambitus almost 



