66 . MR. H.C. CHADWICK ON 
plates of series w are normal until number 46 is reached. Then 
follow a series which, regarded from their inner, tuberculate 
ends, apparently number seven plates, and which exhibit striking 
irregularity of composition (text-fig. 2). It may be that the two 
numbered 47 and 47a respectively compose one but slightly 
imperfect plate, that portion of the suture which traverses 
the large tubercle, and is represented by the dotted line, being 
Text-figure 1. 
Oral end of ambulaerum II. The Arabic numerals indicate the numbers of the 
plates, reckoned from the peristome. a and 6, series in ambulacral area. 
obscure. Number 48 consists of five elements, none of which ranks 
among the primaries, while only one 1s imperforate. The next two 
plates are numbered 49 and 49 @ respectively, on the assumption, 
which the positions of three of the pore-pairs supports, that they 
represent one ordinary plate. The appearance of the next plates, 
numbered 50 and 50a respectively, suggests that they also 
