IMBRICATION OF CORONAL PLATES. 



75 



Text-figs. 32-39. — Method of imbrication of plates in Echini; the figures 32-38 are sj'nthetic, based on studies of 

 Lepideslhes colktii (Plates 69-71). 



32. External view, interambulacral plates are rounded in outline. Column 3 imbricates dorsally and laterally in two 

 directions. Columns 1, 2, 4 imbricate dorsally and laterally in one direction and over ambulacrals. Ambulacral plates 

 imbricate ventrally, bevel under adjacent interambulacrals, but do not otherwise have lateral beveling. 



33. Cross section of the same. 



34. Internal view of the same. Interambulacral plates are hexagonal in outline, angles of imbrication reversed from 

 external view, ambulacral plates with pores in the middle of plates, instead of near the intcrambulacrum as in external view. 



35. Cross section of same. 



36. Optical section of plates seen from exterior. 



37. Optical section of plates seen from interior. 



38. Showing in vertical section direction of imbrication of plates in the interambulacrum and ambulacrum. 



39. Eucidaris tribuloides {La,ma,Tck). Bahamas. R. T. J. Coll., 693. X 4. Base of corona and peristome. On the 

 peristome both ambulacral and non-ambulacral plates imbricate adorally, a condition not known in the corona of any 

 echinoid. 



