Br. F. A. Bather — Eocene Echinoids from Sokoto. 293 



unable to find any other valid species of the genus. Pomel's 

 Cassidulid genus of the same name would, if accepted, belong to 

 the Echinanthinge. Conoclypeus rostratus E. Tate, 1894, was referred 

 by its author to Plesiolampas in 1898 (J.R. Soc. N.S. Wales, xxxi, 

 p. 412), but Mr. J. Lambert in the same year showed that it was an 

 Echinolampas, and named it E. Tatei (Kev. Crit. Paleozool., ii, 

 p. 164). Even if Falceopneustes conicus Dames, 1877, could, as 

 Duncan suggested, be referred to Plesiolampas, it could not possibly 

 be related to the present species, which I therefore propose to 

 describe as 



Plesiolampas Sahara, n.sp. (PI. XI, Figs. 1-5.) 



Diagnosis. — Outline oblong, rounded in front, slightly rostrate 

 behind ; margins tumid ; dorsal surface in transverse profile slightly 

 conoid ; length 100,' width 87-91, height 51-53. Ambulacral areas 

 raised ; petals do not reach margin, posterior pair being most remote, 

 their greatest width from 11 to 16, interporiferous area not quite 

 twice width of poriferous ; pores conjugate and sloping adactinally. 

 Peristome deeply sunk, transversely elongate, eccentric in front. 

 Periproct at extreme actinal margin. Ornament of tubercles sunk 

 in scrobicules of thrice their diametei', irregularly scattered on dorsal 

 surface, more regularly crowded actinally. 



Description of specimen A (B.M. registered E 4824). — Test 

 almost complete, rubbed on left side, fractured in other places. 

 This is the holotype. 



General Form. — Ambital outline approaches an oblong, with 

 gentle curve in front, passing into wide curves on the sides, which 

 are further apart posteriorly, then more tapering behind and very 

 slightly truncate. Margin tumid, very slightly rostrate behind. 

 Actinal surface deeply sunk round peristome, to fully 5 mm. below 

 base-plane,' otherwise gently convex. Peristome slightly eccentric 

 in front, circa 24 mm. from anterior vertical plane. Height of 

 ambitus above base-plane, circa 10 mm. in front, circa 7 mm. behind. 

 Total length, 52-3 mm. Greatest width, 45*6 mm. at circa 28 mm. 

 from anterior vertical plane. Here also is the vertex, 27 mm. above 

 base-plane. Dorsal surface slightly conoid in transverse profile ; in 

 longitudinal profile, slopes down with a sharper and more convex 

 curve anteriorly than posteriorly. (See Text-figures 1-3, p. 294.) 



Apical System. — Slightly anterior to vertex, its centre being circa 

 23 mm. from anterior vertical plane. The 4 gonopores are circular 

 and about equal ; posterior pores 1'9 mm. apart, anterior pores 

 1*25 mm. apart. Oculars and their pores minute and almost indis- 

 tinguishable. As usual in the genus, the 4 genital plates are fused, 



^ In the diagnoses relative measurements are given in hundi-edths of the length ; 

 in the descriptions the actual measurements are given in millimeti'es. Here length = 

 56-60 "5 mm. The radii and interradii are numbered on Loven's system. 



^ The base-plane is the flat horizontal surface on which the denuded test assumes 

 stable equilibrium in its natural posture. The anterior vertical plane is at right 

 angles to this and to the sagittal plane, and parallel to the transversal. The height 

 of the vertex or any other part should always be measured from the base-plane. 



