EOACTINID^. 



201 



my description of Protaster, for I feel that it gives one confidence that we can 

 learn much from the manner in which forms are preserved. 



Among the forms with which we are immediately concerned, we may bring to 

 notice Palasterina and Schuchertia. Palasterina is found in crowded masses 

 (PI. XVII, fig. 3) in a very complete condition, and there can be no doubt that it 

 dug itself into the mud. In support of this we may note that the Devonian form, 



Text-fig. 140. — Drawing' of one of the cotypes of Protaster (Txniaster) spinosus, Billings (Ottawa Mus. 

 no. 1404 6). Ad., adambulacral ; Am., ambulaural ; M.P., mouth-angle plate, x 10. 



P. /oilman ni, is found with its arms lightly flexed over its disc (PI. XVI, figs. 6, 7). 

 Schuchertia on the contrary (see p. 210) probably lived buried just beneath the 

 surface. 



(c) Marginal Plates and their Relationship to the Shape of the Arms. 



It is useful at this point to examine the exact importance of the presence or 

 absence of differentiated marginalia. It seems to me that such plates in fossil 

 forms are a help in determining the original shape of the arms, but of little value 

 in classification. 



