PLATE IV. 



Hemicidaris from the Great Oolite. 

 Fig. 



1 a. Hemicidaris Bkavenderi, Wright, p. 84. Upper surface of the test, natural size. 



b. Lateral view of the same, both the natural size. 



c. One inter-ambulacral plate, and a portion of an ambulacral area, magnified three 



diameters. 



d. An inter-ambulacral plate and tubercle, seen in profile, and magnified three times. 



e. Mouth opening and peristome. 



f. Apical disc, magnified two diameters. 



2 a. Hemicidaris Wrightii, Desor, p. 88. Upper surface of the test, natural size. 



b. Lateral view of the same, natural size. 



c. Apical disc, magnified two diameters. 



d. One inter-ambulacral plate, and a portion of an ambulacral area, with the poriferous 



zones ; magnified three times. 



e. Base of an ambulacral area, showing the semi -tubercles. 



/. An inter-ambulacral plate and tubercle, seen in profile, and magnified twice. 



3 a. Hemicidaris Smithii, Woodward. Under surface, showing the base, natural size. 

 b. Lateral view of the same, natural size. 



Hemicidaris from the Pur beck Beds. 



4 a Hemicidaris Purbeckensis, Forbes, p. 98. Lateral view, showing likewise the 



upper surface, natural size. 



b. Two inter-ambulacral areas, and a portion of an ambulacral area, with the poriferous 



zones ; magnified three times. 



c. Primary spine, natural size, showing the head, ring, and neck, magnified four times. 



d. A portion of the stem, with the same, magnified. 



5 a. Cidaris Smithii, Wright, p. 50. Primary spine, with the surface of the stem, mag- 



nified three times. 

 b, c. Head, milled ring, and neck of the same, magnified four times. 



d. The acetabulum. 



e. A transverse section. 



6 a. Cidaris Ilminsterensis, Wright, p. 31. Inter-ambulacral plate, and a portion of an 



ambulacral area, natural size. 

 b. The same, magnified three diameters. The only fragment of the species I know. 



7 a. Cidaris Bradfordensis, Wright, p. 42. Inter-ambulacral plates, natural size. 



b. The same, magnified two and a half diameters. 



c. Primary spine, found in the same bed of Bradford Clay. 



d. The same, magnified three diameters. 



