442 MR. HEKBERT L. HAWKTNS ON 



However, the difficvilties of a systematic grouping of tlie 

 primitive gnathostomatous Irregularia are not removed, or even 

 lessened, by the additions that have been made to our knowledge 

 of their comparative anatomy. Rather are they increased, for 

 the establishment of affinities between genera leads to greater 

 complexity of classification than that of differences. The 

 Holectypoida are an annectant group, the history of whose 

 evolution is so intimately interwoven with that of the early 

 stages of most of the Irregular orders and suborders, that to 

 frame a purely natural classification would need an impracticable 

 plasticity of diagnoses. This systematic trouble is, however, more 

 than compensated by the phylogenetic evidence that it indicates. 

 In the course of the following work I have endeavoured, while 

 recasting the artificial classification of the systematist, to lay 

 emphasis on relationships rather than on contrasts, and to show 

 the position occupied by the Holectypoida at the JFoundation of 

 the varied structures of the Irregular Echinoids. 



The present essay is the outcome of several yeai-s of study of 

 the group, and contains a summary and amplification of a series 

 of papers (see list at end) that have been published in the 

 ' Geological Magazine.' I have thought it unnecessary to repeat 

 here many of the details described in those papers, so that, except 

 where corrections or additions have been possible, the results 

 arrived at in them are taken for granted. There are, however, 

 descriptions of a number of features that find a place heie which 

 were not dealt with in the shorter papers given here. 



After a brief sketch of the history of the classification of the 

 group, the revised scheme is put forward. This is followed by a 

 morphological comparison of the genera within, and of some 

 genera without, the boundaries of the order ; and lastly, the 

 directions of evolution thus indicated are discussed. 



II. History of Past Classification. 



This part of the paper does not pretend to be a complete 

 account of all the past work that has been done on the group, 

 but it is a summary of the chief systems of classification that have 

 been proposed up to the present time. 



The history of the group may be said to date from 1734, when 

 Klein, in his ' Naturalis dispositio Echinodermatum,' distin- 

 guished " Sectio I, Fibida,^' from the rest of the " Echini caiocysti 

 circularesP The section was diagnosed as follows : — " Echinos 

 fihulares dicimus Cutocystos circa Basis circularis peripheriam 

 Anum, Os in medio aperientes." He included two genera only 

 in the section, Conulus and JDiscoides, both of which are recog- 

 nized to-day, and have been associated with one another in most 

 of the systems of classification. Klein apparently did not 

 knowof any of the other genera included among the Holectypoida 

 in the present paper. 



