158 CRINOIDS. ' 



ossicles ; while the pieces around the anal tube are still 

 smaller, indicating that this structure was very short, and in 

 many cases probably did not project much above the ventral 

 dome. The ornamentation of both also presents a close simi- 

 larity. Some forms of A. orpusculus Hall from the lower part 

 of the Burlington limestone, also show the Physetocrinus 

 physiognomy, but in a much less marked degree. Physetocrinus 

 appears to be the line along which Strotocrinus developed into 

 the unique, short-lived forms which are found only in the upper 

 part of the Burlington. 



With the calyx alone under consideration, Steganocrinus 

 would be referred immediately to Actinocrinus, but the im- 

 mense, narrow, radial extensions from which spring the free 

 arms are certainly distinctive enough for generic separation. 

 Although in this character the genus, at first sight, departs so 

 far from the other groups of the family, it will be seen on closer 

 examination that the departure is only another phase of what 

 is shown in Strotocrinus — -a divergence beginning a little 

 earlier and in a little different direction. 



All through the period of their existence the Actinocrinidse 

 show a decided tendency to increase the distal extent of the 

 rays. In some forms it was accomplished by the simple branch- 

 ing of the free arms, as in Megistocrinus, certain Amphoracrini, 

 and a few Actinocrini; by the lateral expansion of the arms, as 

 in Eretmocrinus; or by the radial extension of the calyx bra- 

 chials, as, notably, in Teliocrinus, Strotocrinus and Steganocri- 

 nus. The number of free arms was thus increased from 20 or 

 30 in the earlier species of Actinocrinus, to 40 to 60 in Teliocri- 

 nus, 100 to 125 in Strotocrinus, and from IfiO to 200 in Stroto- 

 crinus. 



Such, then, briefly sketched, were the stemmed feather- 

 stars in their palmiest days, at a period when the present state 

 of Missouri was covered by the congenial waters of an ancient 

 gulf, vast, shallow, teeming with life. At no other time, within 

 the limits of the region under consideration, were the crinoids 

 at all conspicuous as faunal features. 



