Herbert L. Hawkins — Studies on the Echinoidea. 435 



pyramids. However, where there are teeth there must surely be 

 jaws of some kind, so that it must be allowed that the species of 

 Conulus were gnathostomatous at least as late as the period of the 

 Middle Chalk. 



The case is very different as regards C. albogalerus, the Upper 

 Chalk species. The interior structures are readily accessible owing 

 to the commonly soft nature of the matrix, but hitherto no indication 

 has been found of any buccal armature save the perignathic girdle 

 and the " buccal plates ". In the paper referred to above I accepted 

 the suggestion that the buccal plates might be curiously modified 

 relics of the pyramids, and that the species was edentulous. One of 

 the main purposes of this paper is to express an absolute and complete 

 recantation of this suggestion. The arguments in the next section 

 will, I think, suffice to account for, if not to explain, the peculiar 

 nature of the buccal plates, and the latter description of the presumed 

 fragments of true pyramids recently discovered will afford insight 

 into the reason for their habitual non-preservation in a recognizable 

 condition. 



2. The Buccal Plates. 



One of the most striking features of the Echinoid fauna of the 

 Upper Cretaceous is the massiveness of the test-fabric. Such a genus 

 as Stereocidaris illustrates this point very clearly; but it is most 

 obvious in the Irregular Echinoids. A comparison between 

 Echinocorys and the nearly allied recent genus Urechinus shows 

 an extraordinary contrast in the thickness of the coronal plates. 

 The "Heart-Urchins" of the present fauna are, for the most part, 

 constructed of exceedingly delicate fabric, the test often being as 

 thin as tissue-paper. But Micraster and JTemiaster, the Cretaceous 

 representatives of the "Heart-Urchins", have tests of almost 

 unwieldy thickness. 



As a general rule, the Echinoids of the present day may be divided 

 into two sections ; those whose habitat is exposed, and those which 

 live buried in sand or ooze. The former group comprises most of the 

 Regular Echinoids, and may be illustrated, in an extreme case, by 

 such a form as Heterocentrotus, in which the coronal plates may be 

 as much as a quarter of an inch in thickness. Another type of 

 "exposed" Echinoid is the "Sand-dollar", in which, even when 

 the test is not very thick (as it is in Clypeaster), the whole fabric is 

 supported by pillars and buttresses within. The latter group may 

 be represented by Echinocardium, where the test is exceedingly thin 

 and fragile. The difference in test-thickness is obviously, from one 

 point of view, a mechanical adaptation to the requirements of the 

 habitat. The strong test that will successfully resist the pounding 

 of the waves can be almost dispensed with in the shelter of a burrow. 



There is, however, small ground for believing that the surroundings 

 of such a genus as Ificratter were in any sense tempestuous ; the 

 reverse is more probable. So that the great thickness of the test in 

 the Chalk Echinoids must be due to some other cause. Indeed, it 

 would seem that they underwent excessive calcification quite 

 independent of their mechanical needs; and this thickening was 



