II.—Morphological Studies on the Echinoidea Holectypoida and their Allies

Extract

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 the test-surface of “Pygaster” (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 Pygasteridæ and Holectypinæ 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.