Herbert L. HawJcins — Studies on the Echinoidea. 



Five very small ridges occupy the inner 

 surfaces of the proximal (unpaired) inter- 

 arabulacral plates, never extending beyond 

 the limits of the latter. They are crescentic 

 in plan, have a various sculptured surface, 

 and are always separated from the processes 

 by a space at least as great as their breadth. 

 They may be entirely free [Plesiechinus), or 

 may become more or less involved in the. 

 buttresses when these tend to encircle the ' 

 peristome (^Discoides and Conulus). 



These two sets of structures, according to 

 my belief, are the only ones concerned with 

 the attachment of the jaw-muscles, and so 

 constitute the true perignathic girdle. All 

 other structures entering into the "peristomial 

 ring" are variously swollen portions of the 

 normal plates, adapted for the mechanical 

 support of the processes, the jaws, or the 

 buccal plates (or in some cases of all three 

 sets of ossicles), or for the greater strength 

 of the invaginated part of the corona which 

 encloses the peristome. The true perignathic 

 girdle retains very constant proportions from 

 Liassic to Upper Cretaceous times, its only 

 noteworthy change consisting of a progressive 

 tendency towards increased obliquity in the 

 setting of the processes. 



The Holectypoid girdle, as thus restricted, 

 is extremely primitive. As regards the pro- 

 cesses, it represents a phase reproduced only 

 in the early post-larval ontogeny' of recent 

 Diademoida, and there is some reason to 

 believe that the ridges of non-Cidaroid 

 Regular Echinoids are first developed on the 

 unpaired interambulacral plates. The in- 

 ference may be drawn that the Holectypoida 

 sprang from a Regular stock at a stage when 

 the perignathic girdle had barely progressed 

 beyond the early Cidaroid phase, and that, 

 unlike most of their Diaderaoid relatives, 

 tliey failed to improve upon the almost 

 phylembryonic structure with vphich they 

 were endowed. 



(b) The 2yengnathic buttresses. 



In all of the Holectypoida whose perignathic 

 girdles are known, the processes are supported 

 by buttresses wliich rise from the inner 

 surface of the test. These supports are 

 demonstrably parts of the actual coronal 



o 



o .^ 



•i 2 "= 



.,0)0) 



o' s a o 



S o3 'S 



— ' — c ^ 



CO cc 



U' o 



(D m 



Oi 



^ qj 01 OJ 



fin"-' J--: 



CD +^ ^ 





„ =* > g 



■S ;=; zd '^ 



-g "^ '73 o 



■^ oi 01 2 



^ S s ^ 



•=+-( o» ?H 



rS ^ ^3 O 



bD Q ^ p 



P 0) tC (u 



'. J /3 ^ 

 6 



