200 



BRITISH PALEOZOIC ASTEROZOA. 



is sufficiently strong to wash the stones at the bottom free of mud and so 

 allow attachment of the mussels, etc., or (b) where there are rocks or harbour 

 works sticking up in the water. An Bpifauna is often rich in Echinodermata. 

 Some Asterozoa, e. g. Gorgonocephalus, and, I believe, Brisinga, are important 

 characteristic forms. 



We can use this material to reconstruct the mode of life of the fossil Asterozoa. 

 Consider first the forms which lie buried in the mud. 



(1) These probably will be well preserved, since their bodies will not after death 

 be subject to dissolution by the action of currents or by the carrion-eating animals of 

 the surface of the bottom. 



(2) They may be expected to die occasionally in some characteristic attitude. 

 The surrounding mud would support the arms and so retain the attitude. 



Text-fio. 139. — Drawing of a cross-section through on Echinocardium-Turritella community (kindly drawn 

 for me by Magister Blegvad as a modification of a sketch I submitted to him). A., Echinocardium 

 cordatum (a burrowing sea urchin) ; B., Abra nitida (a Lamellibranch) ; C, Ampltiitra jili/ormis (a burrowing 

 Ophiurid) ; D., Ophioglypha texturata (a surface Ophiurid) ; E., Turritella tercbra (a burrowing Gasteropod) ; 

 F ., a worm ; G., Virgularia mirabilis (an Anthozoan). 



The forms of the genus Protaster and its allies offer most striking examples of 

 this. They are frequently found, as both Ruedemann (1912, p. 90) and Stiirtz 

 (1890, p. 234) have observed, with their arms flexed vertically to the disc (Text-fig. 

 140). It is clear that it is the same attitude as that described by Petersen (1913, 

 p. 26) for Amphiura, which "lies with the body and most of the arms deep down 

 in the clay, but always with the tips of one or more arms stretched out ready to 

 finger any object that comes near and if wanted to draw it down into the clay, 

 where it is swallowed." Petersen also points out that the Amphiurse lie in such 

 quantities that they form a dense net over the sea bottom. The occurrence of 

 Protasti r or its relations in abundance on the same slab, has been noted by several 

 authors, and we now see the explanation that it was when alive a " detritus "- 

 feeder with the same habits as Amphiura. I have noted these facts in advance of 



