H. L. Hau-hins — Buccal Plates in Ecliinocorys. 225 



that species at various stages of its development. They are copied 

 (somewhat diagrammatically) in Fig. 2 for purposes of comparison. 



In the earliest stage (Fig. 2, a) the buccal membrane of 

 Echinocardium is roughly decagonal in outline, and supports three 

 cycles of plates. The inner cycles are rather irregular, but the outer- 

 most cycle is composed of ten large reticulate plates (apparently in 

 a continuous ring) corresponding in position with the ends of the ten 

 ambulacral columns. In ail probability these buccal plates actually 

 represent the primitive terminals of the radial plates, as in a young 

 Diademoid. Their correspondence in position with the ' buccal plates ' 

 of Comdus is noteworthy, but the massive character of the latter 

 ossicles renders their correlation with those of Spatangoids very 

 doubtful. 



In the next stage figured by Loven, the peristome of Echinocardium 

 is seen to be truncated posteriorly by the incipient labrum (Fig. 2, b). 

 This change in shape has disorganized the plating of the buccal 

 membrane, but only to a slight degree in the anterior region, where, 

 for the most part, two large plates of the outer cycle intervene 

 between each radius. The posterior part of the cycle is built of 

 small, irregular plates. In the third stage (Fig. 2, c) the labrum has 

 progressed to such an extent that the mouth opens very near to 

 the posterior margin of the peristome, from which it is separated 

 by a few minute and irregular plates. The disarrangement and 

 multiplication (or perhaps subdivision) of the anterior and lateral 

 plates of the outer cycle is not seriously increased from the degree 

 attained in the second stage. 



In the adult Spatangid (Fig. 2, n) the labrum has so far extended 

 forwards that the mouth (now much enlarged) opens directly against 

 the posterior edge of the peristome, and the only remaining buccal 

 plates are anteriorly and laterally situated. The chief modification 

 that has affected these residual plates is that they have become 

 subdivided, so that three new plates represent two original ones 

 in the two anterior sections. In the postero-lateral interradii, the 

 lateral members of the cycle are still in the original dual grouping. 



The buccal characters, both in peristome-shape and plating, in 

 Ecliinocorys correspond to a stage intermediate between the first and 

 second in EcMnocardium. The primitive three cycles are quite clearly, 

 and solely, represented in Echinocorys, but the ingress of the posterior 

 margin of the peristome, while destroying the inner cycles of plates in 

 that region, has caused a subdivision of the two posterior plates of the 

 outer cycle into three. 



Since the specimen described above is the only one in which the 

 buccal membrane plates of a Cretaceous Spatangoid are known (so far 

 as I am aware), it is interesting to find that the palaeontological 

 evidence is in such direct accord with that derived from ontogeny. 

 It is probable that species corresponding to the later, and perhaps the 

 earlier stages in Echinocardium, will be found among Cretaceous and 

 early Tertiary Spatangoids. 



DECAX)E V. — VOL. IX. — NO. V. 15 



