300 Dr. F. A. Bather — Eocene Echinoids from Sokoto. 



however, is abraded on the actinal surface and on the right posterior 

 quarter of the upper surface ; this is the holotype. B (E 4827) is 

 much more abraded, as well as cracked ; C to (E 4835-484:6) are 

 mostly well preserved, but G is crushed. 



General Form. — The subovoid ambital outline is cut into by the 

 anterior groove to a depth of 23 mm. in A, and 1-8 mm. in B, and 

 is flattened posteriorly by a truncation about 9 mm. wide in A, 

 "6 5 mm. in B. Length of A, 30-5 mm., of B, 25 mm.; greatest 

 width. A, 26-6 mm., B, 22-1 mm. ; greatest height, A, 21-9 mm., 

 B, 18*2 mm. ; distance of vertex from the anterior vertical plane 

 in A, 19'5 mm., but since the test is here eroded, it may have been 

 i'arther back, as in B, where the distance is about 16-5 mm., the 

 ratios to length = 100 being A, 64, B, 66. In determining the vertical 

 anterior plane, ambiguity arises from tlie fact that the specimens are 

 not bilaterally symmetrical, but all project forward more on the 

 right ; one must therefore measure from where this plane cuts the 

 sagittal plane.^ 



Summit depressed. Interradii swollen, all forming a strong 

 convexity, as seen in transversal section, then falling away rapidly 

 to the ambitus, which is at one-quarter the height above the base- 

 plane. In this species the base-plane - is touched solely by the 

 swollen plastron ; and, owing to the asymmetry above noted, 

 ^(luilibrium is attained in these specimens only when they have 

 fallen over to the right ; in measuring, therefore, the sagittal plane 

 must be fixed at right angles to the base-plane. The vertex lies 

 just opposite the middle of the posterior petals; from it the sub- 

 carinate interradiiis curves gently down to the summit of the 

 posterior truncation. 



Actinal surface. — Plastron inflated, especially posteriorly, with 

 a tendency to carination. Margin equably rounded, except posteriorly 

 in median line, in ambulacra I and V, which are slightly depressed, 

 and in anterior sulcus. (PL XI, Fig. 12.) 



Apical System. — Gonopores 4, conspicuous, circular ; anterior pair 

 smaller than posterior, and closer together. Genital plates of each 

 side abut, pushing outwards oculars II and IV; but ocular III comes 

 between the main portions of genitals 2 and 3. Genital 2, however, 

 stretches backwards in a madreporite, between and far behind 

 genitals 1 and 4, also separating oculars V and I, while genital 3 

 sends a broad tongue down to the madreporite. The system is 

 therefore compact and ethmolysian. Ocular pores very small. The 

 oentre of the system, or apical pole, is 13-8 mm. from anterior vertical 

 plane in A, 12-4 mm. in B. (PI. XI, Fig. 7.) In E is an 

 abnormality, gonopore 2 being on the left of ocular III. 



Ambulacra. — Anterior feebly developed ; attains a width of 4 mm. 

 in A. In the proximal 5 pairs of plates the pores are not dis- 

 tinguishable ; in the next 7 pairs they are minute and wide apart; 



' Since noting the asymmetry in Hemiaster sudanensis, I have detected it in 

 specimens of the genus from widely separated localities and horizons. It can be no 

 mere individual abnormality. 



* See footnote, p. 293. 



