224 H. L. Hawkins — Buccal Plates in Echhiocorys. 



they were not actually articulated to the peristome margin, which, 

 in fact, they overlap. Posterior to these four large plates are a pair 

 (on each side) of transversely elongated plates, very obscurely 

 pentagonal, which correspond to the antero-lateral and postero-lateral 

 radii and the included postero-lateral interradii. Three more or less 

 quadrangular plates of about the same size as the last (they are fore- 

 shortened in Fig. 1, a) complete the cycle in its posterior portion. 

 The smallest of the three is on the antero-posterior axis, so that the 

 outer cycle is approximately bilaterally symmetrical. Many of the 

 plates support one or more miliary granules. 



The second cycle is incomplete, as far as the evidence of the 

 specimen shows. It is composed of six elongately hexagonal plates, 

 alternating in position with those of the outer ring. Only the anterior 

 (radially situated) member of this series supports a granule. 



The third cycle is very incomplete, consisting of three plates (with 

 traces of a fourth) of somewhat irregular shape. The central plate 

 has a deep hollow on it, which is probably a pore, but, as the specimen 

 is preserved, bears considerable resemblance to a spheridial pit. 



Owing to the restriction of the two interior cycles of plates to the 

 anterior part of the peristome, the actual mouth opening is driven to 

 the posterior half of the membrane. The smaller plates are to some 

 degree collected round the position of the mouth in a slightly elevated 

 cone. The mouth is situated at the foot of the sharp outward curve 

 of the posterior part of the peristome, but is not in the deepest part of 

 the concavity. 



Comparison tvith peristotnial plates in Echinocardium. The primitive 

 position among the Spatangoida occupied by the Echinocorythidae is 

 well illustrated by a comparison of the plating of the buccal membrane 

 in the specimen described with that of Echinocardium. A characteristic 

 feature in the evolution of the Spatangidae is the gradual increase in 

 prominence of the labrum, and the consequent restriction of the 

 peristomial opening to a crescent shape, when viewed from below. 

 The zonal sequence of the Micrasters, as described by Rowe, illustrates 

 this tendency very clearly. 



Fig. 2. Stages in development of buccal membrane in Echinocardium 

 flavescens. A, in a very young, specimen ; D,, in an adult. Modified from 

 Loven, On ' Pourtalesia ' and Etudes sur les EchinouUes. 



Loven has shown that in the ontogeny of Ecliinocardium flavescens. 

 the peristome is central in the early stages, and retains its position 

 (relative to the anterior margin) throughout growth, its later 

 eccentricity being due to the disproportionate increase of the posterior 

 part of the test. In two of his works ( On ' Pourtalesia ' and Etudes 

 sur les Echindidees) he gives beautiful figures of the buccal plating in 



