29G Dr. F. A. Bather — Eocene Echinoids from Sokoto. 



ambulacrals abutting on a single interambulacral is about 7 half-way 

 down a petal, fewer nearer the apex, and more at the distal end. 

 The arrangement of the interradials on the actinal surface cannot be 

 distinguished. 



Peristome. — Not fully exposed, but enough matrix has been 

 worked away to show that it is about 1"5 mm. in front of the middle 

 line, transversely elongate, narrow antero-posteriorly, and deeply 

 sunken. 



Periproct. — At extreme margin of actinal surface, invisible from 

 above ; longitudinally elongate, oval or subpyriform ; circa 7*7 x 

 4-2 mm. 



Ornament. — Small primary tubercles, varying slightly in thickness, 

 non-crenulate, imperforate, sunk in a scrobicule of thrice their 

 diameter, to such a depth that their summits are flush with the 

 general surface ; irregularly disposed on abactinal surface, both on 

 interambulacral and interporiferous areas, at distances varying from 

 about half the width to twice the width of the scrobicule ; more 

 regularly disposed in rows on actinal surface, very slightly wider, 

 and crowded so that the space between them equals about one-third 

 the width of the scrobicule. Miliary granules, crowded and often 

 confluent, fill all spaces between the scrobicules, and tend to form 

 a slightly defined scrobicular circle ; they cover the ridges dividing 

 the conjugate podial pores ; between periproct and peristome they 

 are larger, and seem here to cover a median tract from which 

 tubercles are almost, if not wholly, absent. (Cf. PI. XI, Figs. 1 and 3.) 



Specimen B (B.M. regd. E 4825).— Much smaller than A, but 

 of the same general shape: length 36-8 mm., width 35 mm., height 

 17'5 mm. The length and height, however, have been much 

 reduced by erosion of the test, which has also caused the excavation 

 around the peristome to appear relatively less than in A, while the 

 periproct can scarcely be distinguished. 



[Specimens C & D (B.M. regd. E 4833 audE 4834).— From Tamaske. C, which 

 is fairly Avell preserved, has length 60-5 mm., width 55 mm., height 32 mm. This 

 indicates a slight increase in relative width and height with age. Otherwise C agrees 

 closely with A. Its measurements are taken into account in the diagnosis. D is 

 a mere fragment, but shows the ornament so much more clearly than the other specimens 

 that it has been selected for figuring (PI. XI, Figs. 1, 3). On the actinal surface, the 

 tubercles are in places more crowded than shown in the figure.] 



Kelations to other species. — Generally speaking, PZesioZamj^as 

 Sahara is stouter, wider, more tumid at the margin, and more loftily 

 domed than any of the Indian species described by Duncan & Sladen. 

 Thus, P. elongata is much narrower and flatter; P. prcelonga is much 

 more elongate, more rostrate, and has narrower poriferous areas in 

 the petals ; P. ovalis is also more rostrate, and has relatively shorter 

 and narrower petals ; in P. rostrata, which has wide petals, scarcely 

 convergent distally, the periproct is more elongate and removed 

 from the margin, while the low test slopes gently from the vertex 

 to the margin and has a slight posterior ridge ; the outline of 

 P. polygonalis, though somewhat reminiscent of P. Saharce, is nearer 

 a pentagon, and is wider in front instead of behind, the low upper 



