54 PEOE. p. M. DUNCAN AND MR. W. P. SLADEN ON THE 



exists, tlie pairs not being so large comparatively as in the 

 younger forms, and with their 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. Linn. 

 Soc, Zool. vol. xix. p. 179.) 



In a specimen attributed to Discoidea cylindrica in the 

 Eritish 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. 



