498 Herbert L. Hawkins — Studies on the Echinoidea. 



7. The Morphogeny of the Cltpeastroid Perignathic Girdle. 



There are two strikingly different types of girdle-structure in the 

 Clypeastroida. In the Fibulariidas, Laganidje, and Scutellidse the 

 "auricles" are situated on the proximal interambulacral plates. 

 In the rest more normal paired processes rise from the proximal 

 ambulacrals. In one respect, however, there is a resemblance 

 between these different structures; there is in all a strong tendency 

 for the prominences to converge towards the interradii. In the 

 Clypeastridae the great development of the proximal ambulacrals, 

 which often meet across the interambulacral, makes such a tendency 

 possible without a dissociation of the processes from their normal 

 foundations. But in the three families named above the proximal 

 interambulacrals are present in unusually large development on the 

 peristomial margin, so that no appreciable convergence of the 

 processes would be possible if they remained on the ambulacrals. 



H. L. Clark has proved that the apparently single interradial 

 " auricles " of the Fibulariid type are in reality double in origin and 

 intimate structure — they are, in fact, dislocated processes which 

 have carried their attempt at interradial convergence to the extreme 

 limit. In the preceding paper of this series I showed how both 

 types of Clypeastroid girdle could be derived from the Holectypoid 

 type. Pygastrides, if its ascription to the Fibulariidse is correct, 

 gives an ontogenetic stage in the migration of the processes from 

 the ambulacra to the interambulacra. The obscurity of Loven's 

 figure in this respect is unfortunate, but it i'-j sufficiently intelligible 

 to show that the processes are as much on the interambulacra as on 

 the ambulacra. PI. XVII, Figs, la, I, c, show in a diagrammatic 

 fashion the possible stages through which this change might be 

 brought about. As to the persistence of a^- interradial element 

 (ridge) carrying the protractor muscles there is, at present, no 

 evidence in either direction. 



Pygastrides affords a valuable simplification to the problem of the 

 derivation of the Clypeastroid girdle from the Holectypoid. Granting 

 the very near relationship between the Clypeastroida and Piscoides, 

 the difference between their perignathic structure is somewhat 

 embarrassing. But if they pass through a Pygastrides-'ph&se, which 

 is simpler in some respects even than that of Pygaster, the girdles of 

 the Fibulariid and Clypeastrid patterns need not be encumbered by 

 the massive false-ridges of Piscoides. The ontogenetic repetition 

 preserves the essentials, and entirely leaves out the individual 

 specializations of the ancestral adults. 



In this respect it is noteworthy that both Pygastrides and the 

 young Echinoneus are entirely without interradial ridges. In the 

 case of the latter, the protractor muscles are fixed to the free edges 

 of the proximal interambulacrals; these being, so far as appears, 

 quite unmodified as special muscle-supports. In the fossil 

 Holectj'poida, as I have shown in this series of papers, there is 

 always some small relic of an interradial ridge on the proximal 

 interambulacrals, minute though these plates usually are. It may 

 Avell be that the absence of these structures in the two young stages 

 is due solely to their youth — they never appear in the edentulous 



