CARADOCIAN OYSTIDEA FROM GIRVAN. , 473 



base of the brachioles. It is bounded by a subtriangular frame formed of the edges 

 of plates 3, 8, probably part of 9, 14, 12, perhaps a small piece of 6, 7, and 2. This 

 frame is broadest at its lower angles (plates 7 and 8) and narrowest just above these 

 (plates 6 and 9). Plates 12 and 14 are separated, but whether 18 enters into the frame, 

 or whether it may have been pushed aside or suppressed, is uncertain, 



§ 457. The periproctal plates are better seen in the larger individual adjoining the 

 main one (PI. V. fig. 58, right-hand), since they are here proportionately larger, nine 

 in 4 sq. mm. They are hexagonal and tend to run in rows, but in neither respect 

 are they regular. In the main specimen the plates near the edge are a little smaller 

 than those in this central and especially the adoral region, sometimes as many as five 

 in a length of 2 mm. In the Oxford specimen some periproctals attain a diameter of 

 ■9 mm., but smaller ones are sparsely interspersed, and as a rule the plates run nine to 

 4 sq. mm. 



§ 458. The number of periproctals, as independently based on the measurements of 

 the two type-individuals just referred to, is estimated as about 1200 in each case. 

 Dr Jaekel (1899, p. 235), basing his estimate on Bone's drawing (Salter, 1866), 

 gives the number as 300-400 ; the estimate of " etwa 1000" given on p. 130 appears, 

 from the reference to pi. 12, f. 6a, to be based on a specimen subsequently made a 

 type of P. anglica and (p. 235) credited with " cirka 1500" periproctals. 



The flexibility of the periproctal integument is inferred from the way in which it has 

 been forced by the surrounding matrix into the thecal cavity and pressed against the 

 inner wall of the antanal face, so that the outlines of the large plates can be discerned 

 beneath it. This is clearly seen in specimen 964, where the antanal plates have been 

 displaced, so that the periproctal integument has been pulled and folded, 



§ 459. The actual Vent is not seen in any of the specimens, but in 964 some of 

 the folds just mentioned appear to converge towards the lower angle bounded by plates 

 3 and 8, at which point the vent occurs in other species. This position is well below 

 the stem-attachment: in the holotype as much as 2*8 mm,, or about the level of the 

 third flange ; in 964, rather lower ; and in 25831 plate 3 reaches to about the level 

 of the fourth flange, and is bent so as to form a channel for the rectum, 



§ 460. The Ornament of the antanal surface consists of granules, about 25 to the 

 square millimetre, arranged in lines parallel to the edges of the plates and concentric 

 with the umbones. These lines of granules may form granular ridges. Crossing the 

 concentric lines are more prominent granular ridges, running from umbo to umbo across 

 the sutures. Occasionally there are subsidiary ridges parallel to these main axial ridges, 

 as well as a faint ridge from the umbo to the angle ; but all these are obscure in the 

 main individual, though plainly seen in a plate (? 6) of an adjacent individual. In 

 specimen 25703 the granulation is much coarser, and the granules are run together 

 mto ridges, between which the surface is smooth ; the axial ridges are the strongest, 

 the subsidiary rhomb-ridges and those to the angles are rather less. 



On the anal face the surface of the plates framing the periproct is ornamented with 



