2 
MUSEUM BULLETIN No. 31. 
Class, Cystoi&ea von Buch. 
Genus, Pleurocystites Billings. 
Pleurocystites laevis sp. nov. 
Plate II, figures 1, 2, 3. 
There occurs at Kirkfield, Victoria county, in considerable num- 
bers, a Pleurocystites which has been a puzzle for several years, and 
which is now put forward as possibly a new species. The specimens, 
though common, are usually none too well preserved, and no single 
specimen retains all the plates in good preservation. But all plates 
of the antanal 1 side have been seen, on one specimen or another, and 
the striking features are the total absence of peetinirhombs and 
surface sculpture, and the smoothness of the antanal side, no umbones 
being present. Tbe arrangement of the plates (see Plate II, figure 2) 
appears to be as usual in Pleurocystites filitextus except that Plate 
XIV laps over onto the antanal side as in P. squamosus. This occurs 
in several specimens, and does not seem to be due to a crack, but a 
real suture. In the specimen figured, the plates of one side are dis- 
placed and Plate II is divided by a crack. Plate XU is broken and is 
not normally so long as shown in the figure, and the suture between 
it and Plate XI is nearly straight (see figure 3). The anal side has 
been observed by breaking a specimen, and though no details are 
visible, there is a large periproct made up of small plates. 
The brachioles are like those of ordinary species of Pleurocys- 
tites. 
The more complete specimen figured is 38 mm. long and 31 mm. 
wide; a small individual is 24 mm. long and 20 mm. wide; still 
another is 34 mm. long and 20 mm. wide. 
Although this is a common and conspicuous fossil it has not been 
commented upon by palaeontologists who have studied the echinoderms 
of Kirkfield, and who may have considered it a worn specimen of 
one of the common forms of Pleurocystites. The plates of many 
of the specimens appear perfectly well preserved, and the sutures 
between Plates I and V, XI and XII, and X and XIV, where the 
peetinirhombs should be, are clearly marked in several. But there 
is no trace of peetinirhombs and it seems incredible that weathering, 
which usually accentuates the striations and pores, should remove all 
trace of them. 
Bather 1 states that in this genus the lower peetrinirhomb between 
Plates I and V is sometimes absent, and that “when this absence is 
correlated with other features, it can scarcely he passed over as an 
individual abnormality.” 
i Trans. Roy. Soc. Edinburgh, 49, pt. 2, No. 6, 1>913 i, p. 4'$1 l. 
