6 Herbert L. Hawkins — Studies on tJie Echinoidea. 



types of Echinonetis and Micropetalon. The Nucleolitoida similarly 

 make their first appearance in the Lias, were extraordinarily abundant 

 and varied in the later Mesozoic, and are reduced at the present day 

 to a solitary and very simple representative of tlieir least specialized 

 family, the l^ucleolitidse. The time of the first occurrence affords no 

 indication of the ancestry of the other three Orders of Irregular 

 Echinoids; and the approximate synchronism of their differentiation, 

 while suggestive of a common origin, may well be but another 

 expression of the parallelism above indicated. Although evidence 

 derived from the study of one series of structures affords no sound 

 basis for the erection of a scheme of phylogeny, it is none the less 

 profitable to examine it as an index of morphogenetic affinity. If 

 the prevalent view of the irreversibility of evolution is correct, such 

 indications should at least show which lines of descent are impossible, 

 and so narrow down the limits of probability, which may be further 

 restricted by similar arguments founded on the observed characters 

 of different sets of structures. It will be in .this sense that sugges- 

 tions as to tlie phyletic affinities of the various forms studied will be 

 put forward in this paper. 



2. The Holecttpoid Girdle. 

 (a) The persistent elements of its st7'ticture. 



Although the perignathic girdles oiFlesiechinus and Conuhis appear 

 to be essentially dissimilar on a casual inspection, the reverse seems 

 to be the truth. If the distinction between the "true" and "false" 

 ridges of the girdle, which I have argued in the two preceding papers, 

 is accepted, the differences between the characters of the " genuine" 

 girdle in the two genera prove to be of so insignificant an order that 

 they would hardly excite surprise if occurring in two species of 

 a single genus. The accompanying diagram (Text-fig. 1), in which 

 the three best-known girdles of the Holectypoida are associated 

 (together with the two chief types of Clypeastroid girdle), shows this 

 fundamental identity more clearly than verbal description could 

 suggest. In the diagram the buttresses and false ridges (where 

 present) are drawn in outline, while the true elements of the girdle, 

 both ridges and processes, are blocked in. The undifferentiated parts 

 of the corona are indicated by shading. The conservatism of the 

 Order could nowhere be expressed more emphatically than by the 

 retention of so physiologically important a structure throughout 

 the Jurassic and Cretaceous stages with hardly appreciable change. 



The essential characters of the perignathic girdle of the Order 

 Holectypoida may be thus described : — 



Ten lath-like processes spring in pairs from the proximal ambulacral 

 plates, and are inclined outwards from the vertical axis.. For the 

 greater part of their length the pairs diverge slightly (i.e. slope away 

 from the perradial line), but at their distal ends they may converge 

 to a small extent (i.e. Discoides), never sufficiently to produce even 

 the semblance of an arch or "auricle". The processes are supported 

 by variously placed and diversely developed buttresses, but are always 

 separated from these by a defined suture, and are composed of more 

 compact stereom. 



