PERISCHODOMUS. 405 



(p. 284). The interambulacral plates in this internal view (Plate 62, fig. 6; Plate 64, fig. 2) 

 imbricate adorally and from the sides toward the center of each area; also they bevel under the 

 ambulacral plates on the adradial suture. All this is just the reverse from what is seen from the 

 exterior (compare text-figs. 32, 34, p. 75). The plates of the interambulacral areas are nearly 

 complete, but are restored in part, as indicated by dotted lines (Plate 64, fig. 2). In four areas 

 the primordial interambulacral plate is in place in the basicoronal row (compare text-fig. 30, 

 p. 70), indicating that there is no resorption of the corona by the advance of the peristome, 

 the same being true as far as known of other members of this family, of Bothriocidaris, and all 

 of the Lepidocentridae. In the second row there are two plates in each area. The initial plate 

 of column 2 is represented on the left in this internal view, which is the equivalent of the right 

 as seen from the exterior, and the same method has been adopted in other cases of internal views 

 (Plate 23, fig. 1). In the third row there are three plates, the initial plate of column 3 being 

 in the center. In the fourth row there are four plates in each area. The initial plate of column 

 4 is on the left of the center in areas A, C, E, and I (which is the equivalent of the right as seen 

 from the exterior), but in area G the initial plate of column 4 is on the right of the center (which 

 is the equivalent of the left as seen from the exterior). The initial plate of column 5 is in place 

 in the fifth row in areas E and I, and is restored in area C, but areas A and G are not shown 

 above the fourth row. This choice specimen shows that in Perischodomus there is no resorp- 

 tion of the base of the corona, and, as a result, the primordial interambulacral plates are in place. 

 It also shows that succeeding interambulacral columns are added rapidly, one in each succeed- 

 ing row as built, the fifth column coming in immediately after the fourth, indicating a markedly 

 accelerated development, much in excess of anything known in the Palaeechinidae, but similar 

 to that characteristic of many of the Lepidocentridae. This acceleration, with the small number 

 of columns attained, which is five, and the early dropping out of two columns dorsally, marks 

 Perischodomus biserialis as a very peculiar type. 



In the Museum of Practical Geology Collection 16,314, is a specimen of this species from 

 Ulverstone, Lancashire, which consists of ambulacral and interambulacral plates with excellent 

 primary and secondary spines. The longest primary spine, which is not complete distally, 

 measures 13 mm. in length, and 1.5 mm. in diameter in the middle of the shaft. The spines 

 are swollen at the base, nearly cylindrical, but taper gradually to the end; smooth throughout 

 with no spinules. If fine striation existed, as is likely, it is worn off. The secondary spines 

 are slender, and measure about 3 mm. in length. This is the best specimen for the spines that 

 I have seen. In the fragment of the ambulacrum, alternate plates are barely pinched off from 

 interambulacral contact, and pore-pairs are biserial. Another specimen in the same collection, 

 16,310, from Coplaw, Clitheroe, Lancashire, consists of dissociated interambulacral plates and 

 spines. The tubercles on the plates are very clear. A third specimen in this collection, 16,31 1 , 

 also from Coplaw, consists of interambulacral plates with spines and pyramids of the lantern. 



