CARADOCIAN CYSTIDEA FROM GIRVAN. 457 



sometimes precisely 3 : 2 : 1, as given by Dr Jaekel for the genus as a rule ; but this 

 rule does not hold, since in P. quadrata, for example, there appear to be more folds 

 in rhomb 1-5 than in rhomb 11-12. 



§ 379. The Primary Pores have been described and figured by Dr Jaekel (1899, 

 pp. 101, 138j. One appears as a slit crossing the suture 18|23 at right angles : this 

 is regarded by Jaekel as the pore of the primary stone-canal (hydropore). The other 

 appears, normally, as a very indefinite hole lower down on the same suture : Dr Jaekel, 

 who regards it as the pore of the parietal canal, probably a gonopore in function, 

 describes it as round and closed by three small plates, but on p. 138 states that these 

 plates are more irregular than shown in his figure. The available material does not 

 enable me to confirm these latter statements. 



§ 380. The enlarged Periproct, of which the limits have already been discussed 

 (§ 371), is filled with small polygonal, usually hexagonal, tessellate plates. These vary 

 in size according to the species. Those of P. Jilitexta are the largest, having a 

 diameter of ca. 27 mm. and numbering from 50 {fide Jaekel) to 70 [fide Billings). 

 In P. squamosa the usual diameter is ca. 1 mm., and the number about 500. The 

 other American species probably fall within these extremes. In the British species 

 the diameter ranges from 1*4 mm. or less, in P. quadrata, down to "4 mm. and less, in 

 P. foriolus, and the number is far greater than in the American species, partly in 

 consequence of the smaller size of the plates, partly because the periproct is more 

 extended. Dr Jaekel estimates the number of periproctals in P. anglica as 1500, 

 and in P. Rugeri, judging from the published figures, as from 300 to 400. My 

 estimate for P. Rugeri, based on the actual holotype, is about 1200 ; and in this 

 respect I find no great difference between P. Rugeri and P. anglica. 



§ 381. In estimating, it has to be remembered that, probably in many species, 

 the periproctals near the margin are smaller than those near the centre. Thus in 

 P. anglica some of the central plates attain a diameter of 1 mm., but those near the 

 frame run 4, or even as many as 9, to the square millimetre (PI. V. fig. 62). They 

 also often become smaller near the vent, and are frequently arranged in lines leading 

 towards that point (PI. VI. figs. 71, 74, 76). 



§ 382. The Vent is surrounded by elongate plates, triangular or rhomboidal, 

 meeting by their apices, as figured by Jaekel (1899, p. 131) for P. squamosa, and as 

 seen in a British Museum example of that species (B 16036). Of these circum-anal 

 plates, those on the lower side near the frame are smaller than those above. The same 

 arrangement is seen in P. quadrata (PI. VI. fig. 71). 



§ 383. If the theca be placed with its anal face upwards, and with the oral pole 

 away from the observer, then the vent lies at the lower right-hand corner, with the 

 circum-anal plates abutting on the frame-plates 3 and 8, but chiefly pressing down on 

 the former. Dr Jaekel (1899, p. 233), referring to the Canadian species and to 

 P. anglica, says that the extension of the periproct has been carried to the utmost 

 limits of possibility. We have, however, already seen that the upward extension is 



