196 Herbert L. Hawkins — Studies on the JEchinoidea, etc. 



wooded heights at Garrisons-on-Hudson, have learned to appreciate 

 the reasons for his success as a leader of men. He is a most 

 unassuming student in the happiest circumstances, ever eager for the 

 promotion of natural knowledge, and watchful to make the best use 

 of all opportunities. To our tribute of admiration we would only 

 add our best wishes for his continued enjoyment of health and 

 strength long to carry on the work for which geological science is 

 already so much indebted to him. 



A. S. W. 



II. — Morphological Studies on the Echinoidea Holectypoida and 



their Allies. 



By Herbert L. Hawkins, M.Sc, F.G.S., Lecturer in Geology, University 



College, Eeading. 



(PLATE XIII.) 



II. The Sunken Tubercles of Discoides and Conulus. 



1. Introduction. 



IN 1914 I published, in this Magazine, an account of " Some 

 Problematical Structures in the Holectypoida ", indicating 

 therein the presence of certain sunken features on jthe test-surface 

 of " Pyg aster'' (Plesiechinus), Coenholectypus, and Discoides. Two 

 series of depressions were described, of which one was wholly 

 sutural in position, while the other was situated on the adoral 

 ambulacral plates, and consisted of more or less sunken tubercles 

 or granules. It is with the latter series of structures that the 

 present paper deals. Although the title " problematical structures" 

 remains appropriate, further work and more refined methods of 

 preparation have made possible a more accurate description of them, 

 and have considerably increased the area of their known distribution. 

 Save for comparisons and passing references, the development of 

 depressions on the tests of the Pygasteridse and Holectypinse will not 

 be considered here. In Jurassic times, when these two groups were 

 at their prime (in this country at least), sunken tubercles were in 

 an incipient stage of evolution, and are in consequence very difficult 

 to distinguish from their normal associates. It is therefore safer to 

 deal first with the well-matured structures, as developed in Upper 

 Cretaceous times. Moreover, the condition of preservation, and 

 especially the character of the matrix, of Chalk fossils, make it 

 easier to clean and stain the specimens without much risk of 

 damage to delicate surface features. A test that has been naturally 

 exposed by weathering is rarely serviceable for study of these 

 structures, and more or less elaborate methods of staining and 

 sectioning are necessary to render them sufficiently clear for accurate 

 description. Small and inconspicuous as the depressions are, they 

 nevertheless appear to have considerable phylogenetic and taxonomic 

 importance. For their physiological value I have no suggestions to 

 offer. 



2. The Ornament of Discoides cylindricus (Lam.). 

 (a) The Inter ambulacra of the Adapical Surface. 

 An examination of an interambulacral plate midway between the 

 apical system and the ambitus shows that the somewhat sparse and 



