1907-8] NOTES ON THE OPHIURIAN GENUS, PROTASTER. 369 
accurate measurements of the extent of the organism. As the disc is 
not preserved in this specimen its dimensions are taken from the other 
examples. The extreme width along the most extended arms is 60 
mm. The diameter of the disc is 15 mm. and this structure is distinctly 
circular in outline. At their proximal ends the arms have a diameter 
of 24 mm., and at their maximum width, about half way down their 
axis, the diameter is slightly more than 4 mm. The dorsal integument 
of the disc is unknown. The ventral side was covered by plates or 
large scales. The preservation is not sufficiently perfect to justify a 
distinct statement in this respect, nevertheless the weight of evidence 
points to a mosaic of thin, sub-hexagonal, minutely tuberculated plates. 
(Fig. 1). 
The ventral side of the arm shows four rows of plates. The two 
inner rows are the ambulacral ossicles; they are distinctly boot-shaped 
and strictly alternating. The two rows are separated by a deep and 
wide ambulacral furrow. The body of each ossicle is about one mm. 
long and the wing is somewhat shorter; they are separated from each 
other by a round interspace which indents the distal border of one 
ossicle and the proximal margin of the one next succeeding. Along 
the length of each ossicle is a slight depression, probably for the 
attachment of muscles. 
Each plate of the outer rows shows considerable evidence of being 
composed of two separate calcifications. The line of suture is very in- 
distinct in all cases, and in most of the plates it is not to be discerned at 
all. Abutting against the “toe” of the “boot” is a stout rounded bar 
which passes outwards and then forwards in a long curve to the outer 
distal margin of the segment. These are the adambulacral plates and 
they bound the ambulacral pore on the outside. Firmly attached to the 
outer margin of this plate is the side plate, which arches inwards 
ventrally so as to protect the soft parts of the arm. While terminating 
in a sharp edge ventrally the dorsal margin is blunt and of nearly the 
same diameter as the plate itself. The plate is ornamented externally 
by two thickened ridges of which the anterior is more pronounced, and « 
there is no evidence that it bore spines. These side plates overlap 
distally, and there is some variation in their shape in different parts of 
the arm. Figures 2 and 4 show the relation of the ventral arm plates. 
The proximal left hand side of the arm shown in the latter figure 
exhibits the side plates in perfect condition ; distally itis more and more 
worn down so that the ventral edges of the side plates are removed and 
