CRINOIDS. 153 



Teliocrinns departs from the type just mentioned in hav- 

 ing a greater nauiber of the lower brachials incorporated into 

 the calyx, thus forming a more or less pronounced decagonal 

 rim just above those of the second order. In this respect it 

 approaches somewhat toward Strotocriuus, but the latter has 

 a very different ventral structure. 



Physetocrinus and Strotocrinus both differ from Actino- 

 crinus in the structure of the ventral side, while the anal open- 

 ing is a simple aperture in the test. The first of these types 

 has the ventral portions of the calyx greatly elevated; the 

 second nearly tlat, while the rim is enormously developed, and 

 the terminal free arms are not given off until the twelfth to 

 fifteenth order of brachials. 



The calyx of Steganocrinus is most like that of the lobed 

 section of Actinocrinus, but the radial extensions are most 

 remarkable, and give rise to a very large number of free arms. 



Inasmuch as the different phases passed through during 

 the known existence of several of the genera mentioned have 

 been referred to already elsewhere, it is hardly necessary to 

 take up here each group separately. It will suffice merely to 

 consider somewhat in detail the geological history of one of 

 the leading generic types — Actinocrinus — which will also indi- 

 cate the general course of development pursued by the other 

 members of the family. 



As yet the genus Actinocrinus is not known before the 

 beginning of the Lower Carboniferous. The forms from this 

 horizon thus far discovered have all a more or less globular 

 calyx, with the arms equidistantly distributed. The ornamen- 

 tation has already assumed two very distinct phases. In the 

 one, delicate ridges or small confluent nodes pass from the 

 central portion of each dorsal plate of the calyx to the center 

 of the adjoining ossicles ; in the other, the ridges are not very 

 conspicuous, and the plates are strongly convex on the outer 

 surface. These two styles of sculpturing continue during the 

 entire existence of the groups. But the first gradually loses 

 its identity, while the second becomes greatly intensified. In 



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