54 PEOE. P. M. DUNCAN AND ME. W. P. SLADEN ON THE 
exists, tlie pairs Dot being so large comparatively as in tbe 
younger forms, and with tlieir arrangement biserial. 
In both instances the expansion of a tubercle-bearing plate 
increases its height ; and it is followed by small low primaries or 
by a demi-plate. The adult specimens conform very much to the 
drawing given by Loven of the peristome of Discoidea conica 
(‘Etudes,’ pi. xiv. fig. 125). 
The peristomial end of the poriferous zones is contracted, and 
a series of tubercles separates the pairs from the position of the 
interradial suture ; so that there were spines in rows up the 
narrow space of the peristomial funnel-shaped tube. The first 
pairs of pores are not visible from without, for they are high up 
in the peristomial tube, and are placed in the processes of a 
structure which is termed the perignathic girdle (Journ. Linu. 
Soc., Zool. vol. xix. p. 179.) 
In a specimen attributed to Discoidea cylindrica in the 
British Museum (No. E 180), which is half-grown, and has not 
become cylindrical in outline above the ambitus, but is simply 
hemispherical, the arrangement of the plates above the ambitus 
is somewhat exceptional. The pores are large and oblique, being 
in simple series and rather distant. The first four plates of a 
certain set are low and broad ; then comes a tubercle-bearing 
primary with a considerable downward expansion towards the 
median line. It is followed by a low primary which is almost a 
demi-plate on account of the diminution of its vertical measure- 
ment near the median line. This loss of size has been due to 
the growth of the primary above, and the pressure has made 
both plates to combine within a geometrical outline to form a 
compound plate. The next plate is a decidedly very low primary, 
and it is succeeded by a primary with a downward expansion 
forming a compound plate with a low primary, which is almost 
a demi-plate. This compound plate is followed by a low 
primary. Just above the margin in this specimen, the growth- 
pressure has altered the shape of the plates in a very instructive 
manner. There is a triplet, and the first plate of it is a large 
downwardly expanded primary ; the next is a low and broad 
demi-plate, for the pressure has blocked out part of a low 
primary near the median line, and a demi has resulted. The 
third plate is a small narrow and low demi-plate ; and this 
was once a very low primary resembling those further up. 
Pressure made it assume the shape of a common small demi, 
