CARADOCIAN CYSTIDEA FROM GIRVAN. 



491 



Plates 

 12 & 14. 



Rhomb 

 10-14. 



Rhomb 

 10-14. 



Rhomb 

 11-12. 



Rhomb 

 1-5. 



Periproctals, 



Average 



Diameter. 



Thecal 

 Outline. 





? 

 •5 



elongate 



kite-shaped 



elongate 



widened, 

 square to 

 rhomboid 



•5 mm. and 

 larger 



quadrate 

 gibbous 



gibha 



so 



)) 



>) 



tends to be 

 widened 



square to 

 trapezoid 



•4 mm., a few 

 larger 



subovate 



foriolvs 



o 



widened 



rhomboid 

 or trapezoid 



widened, 

 trapezoid 



widened, 

 trapezoid 



1 mm. 



quadrate 



quadrata 





>j 



long 

 elliptical 



widened, 



long 

 elliptical 



widened, 



long 

 elliptical 



1 mm. to 

 •3 mm. 



elongate 

 trapezoid 



anglica 



O 

 c3 



)i 



j» 



)> 



absent 



"6 mm. to 

 "4 mm. 



broad 

 trapezoid 



Rugeri 





J) 



kite-shaped 



small 

 triangular 



)) 



1 



elongate 

 ? triangular 



procera 



§ 552. The following tables bring out the relations of the ratios and the shoulder- 

 angle in those American and British species where the necessary measurements 

 are known : — 





American Species. 



filitexta. 



squamosa. 



elegans. 



antieostensis. 



mercerensis. 



Thecal ratio 

 Plate 5 „ 

 Shoulder-angle 



•7 

 •85 



48° 



•8 

 •9 



75° 



•83 



■9 



60° 



■97 

 65° 



•95 

 11 



75° 









British Species. 







procera. 



anglica. 



gibha. 



quadrata. 



Rugeri. 



foriolus. 



Thecal ratio . 

 Plate 5 „ . 

 Shoulder- angle 



•78 

 •74 

 70° 



■81 

 115 

 90° 



•86 



ro9 



95° 



•9 

 1-09 

 100° 



•95 

 1-3 

 90° 



1 



1 



90°-100° 



§ 553. Relations of the British Species of Pleurocystis to the American Species. 

 The differences between these two groups may thus be summarised : — 

 In the British species the periproct is enlarged, and this is effected in four ways — 

 First, it is relatively widened ; in other words, the doublure of the plates forming 

 the sides of the frame is narrower. The loss of strength that would result from 

 this is compensated by the ridges and folds of the surface. Let us compare the 



