H. L. Haivkins — Buccal Plates in Echinocorys. 



223 



absolutely on the long axis of the test. The area of the peristome is 

 not flat, and hardly in any part parallel with the adoral surface of the 

 test. The most deeply sunken part is at the anterior margin, and, 

 with the rest of the test, the edge of the peristome curves sharply up 

 towards the plastron from about the region of its greatest width. 

 This feature is indicated in Fig. 1, b. The actual margin of the 

 peristome is visible in the specimen under description only in its 

 posterior part, being overlapped anteriorly by the peristomial plates. 

 This is probably due to the warping of the membrane on which these 

 plates were fixed. In ordinary specimens, however, the margin is 

 completely smooth, without any trace of sockets for the articulation 

 of the peristomial plates. 



o 



°o= o 



o 



o :o 



Fig. 1. Plating of the buccal membrane in Echinocorys. A, camera lucida 

 drawing, x about 6"5 ; B, sectional view, diagrammatic. 



The Peristomial Plates. The plates of the peristomial membrane 

 are preserved in situ, the slight displacements that they show being 

 probably due to the inward contraction of the membrane after death. 

 They are polygonal, mostly pentagonal, in shape, and seem to be 

 exceedingly thin. The surface of all of them is finely shagreened, 

 in contrast to that of the coronal plates, which are almost smooth 

 between the miliaries. 



The outer ring in contact with the margin of the peristome consists 

 of eleven plates of considerable size and very regularly disposed. 

 The sutures between them are approximately continuous with the 

 perradial and interradial sutures of the test. The four largest plates 

 of this cycle are towards the anterior edge, and correspond in position 

 to the anterior radius and the two antero-lateral interradii. They 

 are pentagonal, and, while opposing straight edges to one another, 

 have a slightly sinuous outline externally. This probably means that 



