494 BR F. A. BATHER. 



original stereom : they lived in a sea where limestone was forming, and they were 

 killed off from time to time by deposits of mud. The British fossils are only imprints 

 in coarse sandstone : they lived and died with hundreds of other organisms in water con- 

 stantly exposed to sand-bearing currents. We need look no further for a foreign body : 

 the sand is enough. To protect these delicate organs against the bombardment of 

 sand-grains, bulwarks were raised higher and coverings thrown across the fields most 

 exposed ; or if that did not serve, the fields were narrowed and the folds extended into 

 more sheltered regions. In vain. The lower rhomb disappeared ; the one to the right 

 is seen on the point of vanishing. And may we not suppose that both the upper 

 rhombs went at last, and that the descendants of the genus, if they survived at all, 

 must have assumed a new form and habits of life unknown to their Canadian ancestors ? 



C. GENERAL CONCLUSIONS. 

 I. Distribution. 



§ 559. The Cystid fauna is no less interesting than the faunas of the Starfish Bed 

 previously investigated. By what it excludes, quite as much as by what it includes, is 

 this fauna peculiar. The whole Order Diploporita and all the commoner forms of the 

 Rhombifera, such as Echinosphaera and Heliocrinus, are entirely absent, though 

 abundant in the Rhiwlas and Sholeshook Limestones and known also from those of 

 Keisley and the Chair of Kildare. Diploporita, such as Sphaeronis, are also common 

 in certain parts of the Leptaena Kalk, while in the Lyckholm beds occur species of the 

 Rhombifera, Hemicosmites and Glaphyrocystis, and it is especially in the Upper Lyck- 

 holm that such appear to be found, though the published evidence on this point is not 

 precise. One is tempted, then, by the easy generalisation that the contrast is between 

 a sand-loving fauna and a limestone or reef-dwelling fauna. It must, however, be 

 remembered that from the probably contemporaneous " Schistes quartzeux " (SI Ic) of 

 Sambre-et-Meuse, Belgium, Prof. Malaise (1900, p. 205) has recorded specimens which 

 I suppose to be of Heliocrinus and Echinosphaera ; and that, on the other hand, 

 Pleurocystis and Cheii'ocrinus are relatively common in the Trenton Limestone. It is 

 therefore probable that other conditions besides mere sandiness affected the composition 

 of the fauna in the Starfish Bed. None the less, the view that the Cystids of the Star- 

 fish Bed were adapted for life on a sandy shore is confirmed by the other main con- 

 stituents of the Echinoderm fauna — the sandstars, starfish, and Edrioasteroids. To the 

 particular nature of the adaptation we shall return ; for the present I merely wish to 

 suggest that the selection of the fauna was due in the main to the physical conditions. 



§ 560. Concerning the Cystidea that do occur in the Starfish Bed, one may repeat 

 the remarks made by Mr Cowper Reed concerning the whole of the Girvan trilobite 

 fauna (1906, pp. 173 et sqq.). " Firstly, there is the occurrence of" a peculiar genus 

 "known at present from no other region," viz. Cothurnocystis. "Secondly," a 



