Nov., 1918] Structure of Agelacrinites and Streptaster 65 



Streptaster reversata Foerste, 1914: Eden shales. Char- 

 acters of preceding Streptasters except that the direction of 

 the tip of ray 5 is reversed as in Agelacrinites. Foerste main- 

 tains that this species is most closely related to A. pileiis 

 among the Agelacrinites. 



Agelacrinites rectiradiatus Shideler. (Published with this 

 article; see description). Figure 30, plate 5, and other figures. 



Discussion of the Skeleton, 

 the peripheral ring. 



The rim of the disc of Agelacrinites is a very noticeable 

 feature. Bounding the central radial and inter-radial spaces 

 are a number of plates of the largest size which stand approxi- 

 mated in a vertical position or nearly so. Outside of these 

 rows of large plates the plates decrease rapidly in size to the 

 edge of the animal where they are very small. 



Of what use were these two divisions in the peripheral 

 ring? Foerste 1914, (p. 407) maintains and with reason that 

 the outer portion was mobile and was the superficial protection 

 of the fleshy margin, which, closely applied to the substratum, 

 held the organism in place. He suggests that the finding 

 specimens loose indicates that they were able to free themselves 

 and reattach. This is paralleled among the Holothuria by 

 the recent P solus fabricii. "In life they attach themselves 

 with the tightness of a chiton to the surf-beaten rocks where 

 they live." — (From letter, H. L. Clark). A specimen collected 

 by C. B. Dyer gives definite evidence that the outer margin 

 was, if not actively mobile, at least adaptable under pressure. 

 A large A. cincinnatiensis is attached close to a Lichenocrimis, 

 the base of some small crinoid. Although Lichenocrimis is 

 often found mis-shapen where conditions did not permit it 

 to develop its circular form, this Lichenocrinus is essentially 

 symmetrical and the margin of the Agelacrinus is deformed by 

 it. The pressure acted on the margin of the Agelacrinite for 

 some little distance beyond the Agelacrinite each way, indicating 

 that the marginal skeleton was somewhat stiff rather than 

 easily pressed inwards. 



According to Foerste the inner larger plates of the peripheral 

 ring make a rigid band. In my opinion while this band may 

 have been rather firmly locked horizontally because the circular 



