320 ROBERT TRACY JACKSON ON ECHINI. 



pore-pairs are uniserial. The structure of the ambulacrum at the mid-zone is intermediate 

 between that of Palaeechinus and Maccoya, and the alternate plates are not usually so com- 

 pletely cut off from the interambulacrum as was observed in M'Coy's type specimen. The inter- 

 ambulacra are wide (Plate 34, fig. 4), with five columns of plates in each area. In the second 

 specimen, on the same slab, there are five columns in one interambulacral area and six columns 

 in two other areas. The plates are hexagonal, or pentagonal in the adradial columns. The 

 hexagons are much wider than high, one at the mid-zone measuring 7 mm. in width and 5.5 

 mm. in height. The ventral area is unknown. Ambulacral and interambulacral plates bear 

 numerous small secondary tubercles, and in the second specimen there are in place small, 

 tapering spines, swollen at the base, and about 3 mm. in length. All five oculars are in place. 

 They are all insert, cover the ambulacra and laterally the interambulacra in part on either side. 

 The genitals are wide, apparently very low for this family, but the ventral borders seem to be 

 pushed under the interambulacra in part, which accounts for the height, and I think also for 

 the absence of visible genital pores. The periproct is exceptionally perfect, the most nearly 

 complete one known in the family. It differs slightly in Plate 34, figs. 4 and 6, but the latter 

 I drew with all care, and it is to be considered a corrected drawing. The plates are numerous, 

 small, angular, rounded up, so that they are higher in the center of each plate than on its 

 sutural margin. They are strikingly close to the character of the same plates in the Recent 

 Cidaridae, and may well stand as representing the typical Palaeozoic periproct, as this is the 

 clearest and most nearly complete specimen in regard to this structure known to me from these 

 ancient formations. This British Museum specimen E 361, is the one from which Duncan (1889, 

 p. 196) described the apical disc with oculars reaching the periproct and with periproctal plates 

 in place. Duncan's second specimen which he says is the same species, and describes as having 

 the oculars excluded from the periproct, may have been the specimen in the British Museum 

 (which was labeled P. sphaericus) which I refer to Lovenechinus lacazei (p. 328; Plate 36, fig. 6). 



A specimen in the British Museum Collection E 4,353, from Clitheroe, Lancashire, is frag- 

 mentary. The interambulacral plates are very large, 11 mm. wide by 8 mm. high, representing 

 a very large individual. Ambulacral plates are preserved only in fragmentary clusters of 

 connected plates, but they are typically alternately primary and nearly, or in some plates 

 quite, occluded, with pore-pairs biserial. . 



The Strassburg specimen of M . sphaerica from Whatley, consists of ambulacral and more 

 or less dissociated interambulacral plates on a limestone slab. All the ambulacral plates meet 

 the middle of the area, and also the interambulacrum, but alternate plates are enlarged or 

 narrowed marginally (Plate 34, fig. 8), the latter not being quite cut off from interambulacral 

 contact, in this respect being like the British Museum specimen E 361. The pore-pairs are 

 biserial. As seen from within (Plate 34, fig. 9), all ambulacral plates are alike, of equal height, 

 and pore-pairs are uniserial. This is the same relation as is shown between the inner and 



