Herbert L. Hawkins — Studies on the Echinoidea, etc. 249 



of that fossil. If it is to be found at all in the typical T. lata-eh&Ik. 

 it is a most exceptional occurrence, while it is found in the first bed 

 above the typical T. lata-ohalk (the " spurious Chalk Rock bed") and 

 occurs freely in the next (the " Vectensis bed "). Micraster is a broken 

 reed for this purpose. Even Howe includes in the zone of II. planus 

 8 feet of chalk (the " Bicavea bed") in which he recognizes the 

 absence of his test Micrasters for that zone, II. Leslcei and If. cor- 

 testudinarium (preecttrsor shape). 



In conclusion I should like to point out that my statements as to 

 the absence or rarity of certain fossils in the zone of T. lata in the 

 Isle of Wight (or elsewhere) cannot be tested by reference to any 

 lists which do, like those of Howe & Jukes-Browne, or may, record 

 the fossils of the "Vecte7isis bed" or corresponding beds under the 

 zone of T. lata. I have, for my own part, relied on my general 

 experience of typical T. lata-ch&Ik in the South of England, 

 supplemented by an examination made expressly for this purpose of 

 the exposures at Compton Bay and Culver Cliff on occasions when 

 they were in excellent condition. 



III. — Morphological Studies on the Echinoidka Holkctypoida and 



their Allies. 



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

 College, Beading. 



III. Some variations in the Structure oe the Apical System of 



SOLECTTPUS. 



(PLATE XVI.) 



1. Introduction. 



STRUCTURAL variation, unless due to injury or disease, may be 

 regarded as a sign of racial health. The plasticity that allows 

 or encourages deviation from the normal is an indication that the 

 race is still young and vigorous, and that the deadening and 

 ultimately fatal phase of stereotyped "perfection" has not been 

 reached. The younger the race the more variable are even vital 

 structures. It may be surmised that the several stages of race-life 

 can be classified by the quantity and quality of variability shown. 

 Indeed, variation, however induced or employed, is the determining 

 force of evolution. Variants must therefore be considered as tentative 

 and experimental efforts whose direction is parallel, or coincident, 

 with the trend of evolution of the group. They can be used as 

 indices of phylogeny that afford evidence of the ancestry, and also 

 of the posterity, of their own and related stocks. 



The recent work of R. T. Jackson on the relative positions of the 

 ocular and genital plates in the apical systems of many Regular 

 Echinoids, has brought to light abundant and convincing proof that 

 a species consists of a "norm" surrounded by a series of regressive 

 (or arrested) and progressive variants. 1 It has also been shown 



1 See especially "Phylogeny of the Echini " and "Studies of Jamaica 

 Echini". 



