PLATE XI. 



CiDARis SERRiFERA, Forhes, 1850. 



From ike White Chalk. 

 Fig. 

 1 a. Test and spines, natural size. Collection of Henry Willett, Esq., F.G.S. P. .51. 



b. Under surface of the same test, natural size. 



c. Lateral view of the same, natural size. 



d. Inter-ambulacral plate, ambulacra, and poiiferous zones, magnified tliiee diameters. 



e. Liter-ambulacral spine, natural size. 



/. Stem, neck, and head of the same, magnified three diameters. 



(f. Portion of the stem, highly magnified, to show the serrated spines and intervening 



sulci. In the cabinet of Dr. Wright. 

 h. Ambulacral spine, highly magnified. 



2. Portion of a spine of C. serrifera, magnified. Collection of Professor Teunant, F.G.S. 



3 a. Spines of Cidaris. British Museum. 

 b. One of the spines, magnified two diameters. 



4. Cidaris Dixoni, Cotteau, 1862. Inter-ambulacral spine, natural size. Proiu tlie 



Grey Chalk of Dover. Collection of Henry Willett, Esq., F.G.S. 

 a. A section of another specimen, to show the cavity in the stem. 



5. Cidaris pleracantha, Agassiz. Spine, natural size. British Museum. 



6. Spines of Cidaris, natural size, and magnified twice. British Museum. 



