PLATE IV. 



Cidaris clavigera, Konig, 1822. 



From the White Chalk. 



Fig. 

 1 a. Two tests, with spines, in one block of chalk, natural size. From the late Mr. 



Taylor's collection, now in the cabinet of Dr. Bowerbank, F.R.S. P. 48. 



b. Inter-arnbulacral plate, ambulacra, and poriferous zones, magnified three 



diameters. 



c. A large tubercle, and circle of areolar granules, magnified. 



2, The apical disc, with the ovarial, ocular, and anal plates in situ, magnified two 

 diameters. 



3 a. A large inter- ambulacral spine of a typical form, natural size. 



b. The same, magnified two diameters. 



c. A portion of the neck of the same, magnified three diameters. 



4 a. A large spine of an abnormal form, natural size. 



b. A large spine of an abnormal form, magnified two and a half diameters. 



c. A portion of the neck of the same, magnified three diameters. 



5 — 20. Different forms of inter-ambulacral spines, from a series in the collection of 

 Dr. Bowerbank, P.R.S. 



21. The neck, milled ring, head, and acetabulum of the specimen figured at' 3 a, 



magnified four diameters. 



22. A small ambulacral spine, highly magnified. 



23. Another ambulacral spine, highly magnified. 



