458 



MR. HERBERT L. HAWKIXS ON 



are so small that, in spite of the relative minuteness of the peri- 

 stome, the margin is actually less notched than in Pygaater. In 

 Conulus the slits ai'e only just distinguishable on the thickened 

 rim of the peristome. In Fyrina^ and in the Echinoneidee gener- 

 ally, they seem to be altogether absent. 



The concavity of the adoral surface is found to correspond 

 fairly closely with the development of the branchiae. In Pygaster^ 

 Anorthopygus^ and Holectyptis, the adoral surface is mai-kedly 

 concave, and the peristome is situated in an additional hollow in 

 the centre. In Biscoidea the surface is almost flat, and yet the 

 peristome is deeply sunken. In Conulus, on the other hand, the 

 mouth is practically flush with the test-surface. In the Regular 

 Ectobranchiata the length and stoutness of the radioles are suf- 

 ficient to keep the test permanently raised above the rock surface. 

 The branchiae are by this means kept free from the danger of 

 becoming bruised or fouled by contact with the ground. In 

 the Holectypoida the radioles Avere certainly not so strong as, and 

 probably of far less length than, those of an average Regular 

 Echinoid. As a consequence, the adoial surface would be usually 

 very near to, if not in actual contact with, the rock surface. tSuch 

 a, condition would have a disastrous effect on such delicate oi'gans 

 as the external branchiae. It seems possible, therefore, that the 

 concavity of the adoral surface of the test is a device for sheltering 

 these structures. In the case of Discoidea, where, for purposes of 

 internal consolidation, the lower part of the test is flattened, the 

 region of the peiistome is sunk to a proportionately great degree 

 to afford this shelter for the branchiae. In Comdus, whei'e the 

 branchiae were practically negligible in size, and probably in 

 fvmction also, no such precautions were necessary. For gnatho- 

 stomatous forms, like the Holectypoida, which were presumably 

 not wholly, or even chiefly, microphagous, this depression of the 

 peristome would appear to be disadvantageous for the capture of 

 food ; and only the safety of the equally essential process of 

 respiration could warrant such a development. However, it must 

 be remembered in this connection that the Clypeastroids, in a 

 considerable number of cases, possess a re-entrant peristome 

 without any external branchiae. They have grooves on the adoral 

 surface converging on the mouth, which may counteract what 

 seems to be an unprofitable structure. Moi'eover, among them 

 the indentation of the peristome is in all proba.bility connected 

 with the accommodation of the large jaw-apparatus. 



The Holectypoida oflTer no satisfactory evidence as to the 

 relation between the peristomial and petaloid branchiae. In the 

 Upper Jui-assic Pygasters (e. g., P. {Megapygns) maGrociiphus) 

 there is a marked tendency towai'ds petaloid structure in the 

 adapical parts of the ambulacra, but the branchial slits are as 

 well developed as in any of the earlier species. As the genei'a 

 are ti'aced to the Upper Cretaceous, there is an ii'regulai' but 

 frequent tendency seen for the adapical ambulacral pores to 

 become dissimilar, but there is neA^er anv contrast sufiicient to 



