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



conclusively that the actual position of the norm between the 

 extremes of variation differs in separate localities, resulting in local 

 "varieties" which might be expected to extend their divergence 

 until a distinct series of "species" was differentiated. A sufficient 

 compilation of records would make possible a fairly confident 

 prediction as to the main characters of these future species. 



In the study of palseontological material there is often an 

 opportunity to make actual observation of the ancestry and 

 descendants of a particular species, and so it becomes possible 

 to correlate the variations of that species with the known trend of 

 evolution in the group to which it belongs. Conversely, an analysis 

 of the variations may give suggestive evidence of relationships. 

 I have been able to examine a considerable number of specimens of 

 the common Inferior Oolite form, Holectypus hemisphcericus, from two 

 localities in South- Western England. The results, in so far as they 

 depend upon the structure of the apical system, seem sufficiently 

 striking to be worthy of record. It must be remembered, however, 

 that these results are based upon a study of some scores of 

 individuals, while Jackson's are derived from hundreds or even 

 thousands. The percentage calculations would be liable to con- 

 siderable change when made from larger numbers. 



In 1912 I showed, in this Magazine (Dec. V, Vol. IX, p. 8), the 

 extreme diversity of structure met with in the apical system of 

 the Holectypoida. Within the limits of the order there exists 

 a record of the various methods whereby the readjustments, 

 consequent on the backward migration of the periproct, were 

 effected. The phylogenetically recent date of this disorganization 

 rendered the apical system peculiarly sensitive to variation, and 

 it was only in the later members of the order, and in descended 

 stocks, that any fixity of structure was attained. The Bathonian 

 species of Holectypus, though far in advance of Plesiechinns (in the 

 repair of the apical system), are sufficiently near in time to the 

 presumably Liassic origin of the order to exhibit much instability 

 of apical structure. It would be interesting to examine a large 

 series of H. depressus (the Inferior Oolite contemporary of H. hemi- 

 splicericus), so that a comparison could be made in this matter 

 between the two strikingly distinct species. If any Cotteswold 

 collectors who have a series of H. depressus would enable me to 

 examine their specimens, I should be very grateful for the 

 opportunity. Large numbers from one horizon and locality are 

 required for useful results. 



2. Holectypus hemisph^ericus. 



This well-known species is extremely abundant in the upper part 

 of the Inferior Oolite (particularly in the schlwnbachi-hemera) of 

 Dorsetshire. Its distribution is peculiar. As llichardson has shown, 

 it is scarce, and often absent, in the Cotteswold district, flourishing 

 only to the south of the Mendip axis. Its place in the more 

 northerly region is taken by the small H. depressus, which, in its 

 turn, is excessively rare in the south. In many parts of Dorsetshire 

 H. hemisp/uericus and Pygorhytis ringens vie with one another in 



