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



are smaller. They appear to have covered thickly the whole dorsal 

 surface. On the actinal surface they are rather larger, and, on the 

 periplastronal ambulacra, are interspersed with secondaries. The 

 bosses are low, rounded, and where well preserved finely crenulate. 

 No signs of perforation can be traced, but this is not enough to 

 prove that the tubercles were not slightly perforate. Traces of the 

 peripetalous fascicle are clearly seen in A, crossing the anterior 

 groove at 15 mm. from the apex, and at the end of petal V, well 

 beyond the poriferous area ; also, less clearly at the end of petal IV. 

 There is not the smallest trace of any other fasciole, but all regions 

 where such might be expected are crowded with tubercles. 



[On the other hand, all the Tamaske specimens, except the crushed and abraded 

 G and N, show the lateral fasciole distinctly. It is linear, but incised, continues the 

 line of the peripetalous fasciole from the ends of petals II and IV, till opposite 4;he 

 ends of petals I and V ; here it dips to nearly the base of the posterior truncation. 

 The posterior tract of the peripetalous fasciole leaves the main line about the middle 

 of interambulacral series lb or 4« respectively, rims parallel to petals II and IV, 

 then parallel to the lateral fasciole, till it reaches the ends of petals I and V, whence 

 it cuts straight across the carina of 5 (PI. XI, Fig. 13).] 



Eelations to other species. — ThenumberofspeciesofiZem«as/er 

 is large, but the majority are either distinctly ethmophract, or, if 

 ethmolysian, have less than 4 gonopores. Taking those that remain, 

 it appears that our species differs from them all in the greater 

 elongation or narrowness of the adult test, and from most also 

 in the deep excavation of the ambitus by the anterior sulcus, while, 

 from the few that have this character, it differs in the shallowness 

 of the same groove near the apex. One or other of these features, 

 combined with others mentioned in the diagnosis, separates it so 

 definitely from all species of Trachyaster type, that detailed com- 

 parison is needless. 



In general form the Sokoto fossils approach more nearly some 

 species of Linthia, and, in view of a possible confusion, the differences 

 may be pointed out. Thus Zinthia bisidca Peron & Gauthier has 

 petals more equal in size and a smaller posterior truncation ; Cotteau's 

 figure of this (Pal. Franc, tom. cit., pi. Ixxx, fig. 5) shows neither 

 the heteronomy said to characterise Linthia nor that found in 

 Hemiaster. In X. Cotteaui Tournouer and Z. dubia Cotteau, the 

 posterior truncation is vertical and the periproct invisible from 

 above ; in the latter also it is transversely elongate. L. Pomeli 

 Cotteau is rather elongate, but the angle subtended by ambulacra II 

 and IV is greater, and petals I and V are much longer. Z. Diicrocqui 

 Cotteau is somewhat elongate, but width : length : : 95 : 100 ; the 

 posterior truncation slopes downwards and inwai'ds so that the 

 periproct is invisible from above ; the pores are circular and not 

 conjugate ; for this species also Cotteau's figure (tom. cit., pi. Ixvi, fig. 3) 

 does not show ancient heteronomy. Z. sindensis Duncan & Sladen 

 has a greater slope to its posterior truncation, and width : length : : 

 103 : 100. No other species of Zinthia seems to present greater 

 resemblances than do those mentioned. 



Evidence as to age. — The genus ZTemiaster, in its typical 

 ■ethmophract form, ranges from Cretaceous times to the present day, 



