VI. Pyrgocystis. 



1915 57 



a shallow- water formation, but not so rich in life as the Stictoporella 

 bed, and especially poor in sedentary organisms. No firm base was 

 afforded for the turret, which therefore seems to have tapered off 

 to a pointed end thrust in the soft sand, while upwards it grew out 

 of the reach of a bombardment by sand-grains (cf. " Caradocian 

 Cystidea from Girvan ", § 558). By the domed elevation of the oral 

 face, the accumulation of sand-grains over the subvective system was 

 prevented. The water was not so rich in lime, and parasitic enemies 

 were not so numerous, wherefore it is probable that the reason why 

 spines have not been preserved is because they never were developed 

 in this species. 



The Wenlock Shale of Shropshire, like the contemporaneous Bed c 

 in Gotland, is a very fine-grained shale, apparently deposited in calm 

 waters, in which lived a fairly numerous but rather dwarfed fauna, 

 rich in ostracods and in small brachiopods and trilobites. There was 

 no good fixation for the turrets, which seem therefore to have been 

 rounded off at the distal end, and presumably stuck in the ooze, but 

 may possibly have been attached to seaweeds. Frequently, it seems, 

 they did not stand upright, and therefore grew in a slight curve. As 

 a rule, relative want of lime made their plates thin and of a loose 

 texture as now manifested in their dark colour. This also did not 

 encourage the production of spines, which probably were absent. The 

 differences between the various Wenlockian species do not seem to be 

 of much physiological importance. 



The results of the present Study may now be summarised in the 

 following Diagnoses. 



Pyrgocystis 1 n.g. 



An Agelacrinid with a subvective system of five broad straight rays 

 mounted on a subcylindrical turret of adorally imbricate thin wide 

 plates, which are not markedly different from those of the interradial 

 areas of the oral face. 



Range. — Lower Ordovician to Middle Silurian. 



Genotype. — P. sardesoni. 



P. SARDESONI n.Sp. 



A Pyrgocystis with diameter of turret equal all the way up, and 

 about two-thirds its height, which is about 14 mm. ; turret-plates in 

 about 48 tiers of 11 plates in each, irregularly alternating and 

 nowhere forming the complete 22 columns ; each plate is from 

 two to three times as wide as high, and is shaped like a segment of 

 a flat ring. Not more than one-third of each plate is exposed. At the 

 distal end of the turret the plates are almost horizontal ; towards the 

 proximal end they gradually approach the vertical, and finally merge 

 into the interradial plates of the oral face, which is approximately 

 horizontal. Spines are borne by all plates, those on the turret-plates 

 being minute and not readily preserved. 



Holotype— Brit. Mus., Geol. Dept., E 16232. 



1 irvpyos, a tower. 



