436 Herbert L. Hawkins — Studies on the Echinoidea. 



therefore likely to affect other stereom-structures besides the actual 

 corona. The buccal-menibrane plates of Echinocorys, as Lambert and 

 I have described them, are far more robust than those of similarly 

 sized Spatangoids of the present day, although far inferior in 

 thickness to the coronal plates. The periproctal plates of Discoides 

 are equally strong. 1 



In a specimen of Conulus alboyalerus, the interior of which is 

 almost filled with flint, there are numerous thick, granular plates in 

 the mealy chalk within the periproct. They are like the buccal 

 plates in every feature except that of shape, and I have not the 

 slightest hesitation in identifying them as the plates of the periproct- 

 membrane. Since the periproctal plates are so enormously thickened 

 in this species, it is only to be expected that the peristomial ones 

 should be in a similar condition. There is, therefore, good reason 

 for believing that the buccal plates of Conulus are merely the plates 

 of the buccal membrane, and that their strange proportions are but 

 an advanced expression of the Upper Cretaceous " mode ". 



[Note inserted in proof, September 14, 1917. — The preceding 

 paragraphs were written in July of this year. The Editor has 

 kindly drawn my attention to two recent papers in this Magazine 

 which include references to the phenomenon of " super-calcification " 

 in Cretaceous forms. One, by "W. D. Lang, deals with Polyzoa, 

 and the other, by C. T. Trechmann, is concerned with Mollusca. 

 The Editor has also reminded me of the extraordinary superfluity 

 of shell-substance developed in the Rudistacea in the same period. 

 In some cases it is easy and probably correct to ascribe the secretion 

 of apparently unnecessary mineral matter to phylogerontic over- 

 specialization (e.g. Parlcinsonia dorsetensis from the Bathonian, and 

 Clavella longoeva from the Bartonian). But in the case of the 

 "Heart-Urchins" above mentioned, such an explanation seems 

 impossible. Hemiaster, Ificraster, and Epiaster represent the pro- 

 gressive pioneers of the Spatangina, not degenerate and superannuated 

 relics ; and a quality that affects a large proportion of a fauna, 

 irrespective of the phylum or phylogenetic phase of the individuals, 

 must surely have originated from some more comprehensive and 

 fundamental cause. However, this is not the place for a discussion 

 of the problem.] 



If the foregoing interpretation of the buccal plates is correct, an im- 

 portant corollary follows. Loven showed that the young Echinocardium 

 flavescens, where the outline of the peristome is roughly circular, has 

 ten elliptical plates on the buccal membrane, arranged in a complete 

 cycle near the circumference of the aperture. In 1912 1 indicated 

 the close correspondence in most characters between the early 

 ontogenetic stage of the buccal plating in Echinocardium and the 

 " buccal plates" of Conulus; but hesitated to correlate the two sets 



1 The same remark applies to the periproctal plates described by F. J. North 

 (Ann. Mag. Nat. Hist., December, 191-5). His specimen is obviously a species 

 of that Upper Chalk Caratomid genus variously called Echinocomts (Desor, fid. 

 Lambert), Pironaster (Schlueter), or Conulopsis (Hawkins). Anyhow, the 

 horizon from which it was collected, and all the particulars published about it, 

 show clearly that it is not a Discoides. 



