CARADOCIAN CYSTIDEA FROM GIRVAN. 477 



level the sides of the theca appear to slope gently towards the oral pole, so that the 

 outline was probably a trapezoid, truncate at the stem-facet and the oral end. The 

 actual height of the theca cannot be measured, but a comparison with P. Rugeri may 

 be based on the following measurements : Height from stem to umbo of plate 10, in 

 25704, ca. 17 mm., i.e. ca. "63 of width ; in 6, 20 mm., i.e. '77 of width ; in P. Rugeri, 

 •61 of width. These ratios, no less than the gentler slope of the sides and the greater 

 width between the upper pectinirhombs, indicate that the theca was relatively more 

 elongate in P. anglica. 



§ 471. This conclusion is borne out by the measurements of individual plates, all 

 of which are relatively narrower than in P. Rugeri. Of these, plate 5 in particular has 

 always been found a safe index to the general proportions. In 25704, plate 5 has a 

 greatest width of ca. 9 '2 mm., a height of 8 '4 mm. ; in 6 its shape is distorted, but 

 measurements along the ridges give width 12 '6 mm., height 107 mm. The ratios of 

 height to width are therefore : in 25704, '91 ; in 6, "85 ; in P. Rugeri, -77. 



§ 472. The ratio of thecal width to height given in the diagnosis, viz. '8, is the 

 mean of the ratios severally calculated from the various measurements just given. 

 The same ratio, if calculated from Jaekel's estimated measurements, would be '83 ; in 

 P. Rugeri it is '95. 



§ 473. Plate 6 is in specimen 6 partly covered by plate 5, through which its outline 

 can be dimly apprehended ; in 25704 it has a width of ca. 7*5 mm., a height of 8*6 mm. 

 Plate 8 is not clearly seen. Plate 9 in specimen 6 has a width of 9 '4 mm., a height 

 of 11 "5 mm. ; in 25704, a width of 8 mm., a height of ca. 9 '5 mm. 



§ 474. Comparison of Dr Jaekel's fig. 6 with our PI. V. fig. 60 and text-fig. 7Q 

 will save much detailed criticism. In this specimen, as in 25704, an unusually large 

 amount of plate 7 is visible on the antanal face, and here it seems to stretch between 

 plates 2 and 1 below, and 6 above, right up to plate 5. The disturbance of the plates 

 renders it difficult to reconstruct their precise outlines, and my interpretation does not 

 quite agree with Dr Jaekel's figure. We agree, however, in representing some 

 unusual plate between 1 and 6. In this connection the peculiar outline of plate 7 in 

 25704 is noteworthy : it has a side indicating abutment on a plate either invisible or 

 no longer present. It is just possible that both specimens had this peculiar structure. 



I 475. Plates 2 and 3 do not descend so far below the stem-facet as in P. Uageri, 

 but the precise level they reach is not clearly seen. 



I 476. There are three Pectinirhombs in the usual position, all with a thickened 

 border raised well above the thecal surface, and all apparently widened across the 

 suture as in P. Rugeri, that is to say, to a far greater extent than represented by 

 Dr Jaekel's draughtsman. 



§ 477. Connected with the presence of pectinirhomb 1-5, the umbo of plate 5 

 projects slightly more than in P. Rugeri. In specimen 6, this rhomb has a length 

 of ca. 7 mm., a width of 1-6 mm., and contains about nine folds. In 25704, this rhomb 

 seems to lie almost on the suture l|4, and to pass downwards to the stem, an appear- 



