MEEKECHINUS. 445 



as far as observed, ambulacral and interambulacral plates ai'e essentially the same as they are 

 on the dorsal side. A few ambulacral plates are clinging to the oral face of the lantern and are 

 probably peristomal, but they are not sufficiently continuous or complete to be certain. Am- 

 bulacral plates only on the peristome is the character to be expected, as that is the structure of 

 the allied Lspidesthes (Plate 68, fig. 3). 



Dorsally, the apical disc is in place and, though not perfect, is well preserved. In the holo- 

 type (Plate 75, fig. 6) the apical disc measures about 9 mm. in diameter through the axis III, 

 5, which is proportionately about 16 % of the diameter of the test (pp. 87, 362). The oculars 

 are small and imperforate, ventrally covering the ambulacra and laterally in part the inter- 

 ambulacra on either side. The youngest interambulacral plates of each area are in contact 

 with the oculars of either side, as seen best in interambulacrum 2, Plate 76, fig. 6. Oculars 

 II and III are exsert, being shut out from the periproct by the contact of the associated genitals. 

 Ocular V is probably insert, I is wanting, and ocular IV is doubtful. The genitals are all present. 

 Genitals 1, 3, 4, and 5 are all medium sized, rounded in outline, and of about equal size. Each 

 of them bears a small perforate primary tubercle with a scrobicule and smaller secondary 

 tubercles similar to those on interambulacral plates. Genital 1 has five pores, 3 and 4 have 

 four pores each, and genital 5 has three pores. Genital 2, the madreporite, is very much larger 

 than the other genitals. It has numerous fine madreporic pores as in a Recent Cidaris, and is 

 the clearest madreporite seen in the Palaeozoic (p. 172). There are in this plate only two 

 genital pores. It bears no primary but a few secondary tubercles scattered amongst the madre- 

 poric pores just as can be seen in a modern cidarid, as Eucidaris trihuloides (text-figs. 62-69, 

 p. 98). A few small periproctal plates are present, rounded in outline, but somewhat angular, 

 and in general similar to the periproctal plates of other Palaeozoic Echini and modern cidarids 

 (p. 174). 



An excellent lantern is preserved in one of the specimens (Plate 75, fig. 8; Plate 76, fig. 7). 

 It is inclined, with pyramids wide-angled as in other Palaeozoic Echini. The most important 

 feature of the lantern in this species is the teeth, which are grooved on the inner face, vertically 

 ribbed on the outer face, and, as a very important character, are distally sharply and deeply 

 serrate as seen clearly in two teeth figured (Plate 75, fig. 8; Plate 76, fig. 7). This distal serra- 

 tion is a unique character in Echini as far as I am aware. Taking up the other characters of 

 the lantern, in area E the two half-pyramids are in contact on the line of the interpyramidal 

 suture. In this area it is seen that the foramen magnum is very deep, more so than in any other 

 Palaeozoic species seen (compare Plate 27, figs. 4-6; text-figs. 207-213, p. 184). On the left 

 half-pyramid the suture exists for the epiphysis, but that structure is wanting in this area. 

 The epiphysis is in place, however, capping the right half-pyramid of area C, and above it 

 lies the tip of a somewhat displaced brace. Due to a local twist, the outer side of the left 

 half-pyramid of area A is seen, and this face has plicate ridges for the attachment of interpyram- 



