474 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



enlarged, and evidently movable, pinnule — apparently an oral 

 pinnule. 



Another interesting parallel is that the ambulacral ossicles of 

 Chinianaster are solid, block-like structures, like the arm- joints of 

 crinoids. Chinianaster had pointed tube- feet (preserved in some of 

 the fossils), and they were carried in basin-shaped depressions on the 

 lower side of the arm, one basin between each adjacent pair of ossicles 

 (fig. 9, D). In the more starfish-like somasteroids, however, there 

 grew out a lateral wing from each ossicle, and the basins for the tube- 

 feet became a mere shelf, which sank inward between successive 

 wings. In the Ordovician genus AmpuIJnster, and to a lesser ex- 

 tent in the contemporary somasteroid ViUehrunaster, the shelf led to 

 a definite internal opening, labeled P in figure 9, F. The pore was 

 evidently for an internal extension of the base of the tube-foot, a 

 small reservoir called the ampulla. Platasterias has a small ampulla, 

 and starfishes generally have a large one. The Ordovician somaster- 

 oids Villehrunaster and Ampullaster also have the innermost vir- 

 galium somewhat enlarged, forming a definite adambulacral plate, 

 as in starfishes; and the outermost virgalium in these genera was 

 transformed into a marginal plate. Thus, many features of modern 

 starfishes were already developed in some of the lower Paleozoic 

 somasteroids. These facts have only recently become known, the 

 earlier descriptions of fossil somasteroids having been very inade- 

 quate. It is clear that no sharp boundary separates the starfishes 

 from the somasteroids, and that the mosaic-like pattern of the skel- 

 eton of modern starfishes arose from the frond-like pattern of the 

 somasteroids. 



Kefer again to figure 7, and note the three main phases in the 

 transformations of the gradient patterns, which are simplified in the 

 diagrams, so as to stress the main features. 



Figure 15 gives a representative series of arms of genera illustrating 

 these three main patterns, though it also shows that there are no hard 

 and fast boundaries between them. In figure 15, C {Chinianaster) , 

 and D {Platasterias)^ there are dominant transverse gradients and 

 petaloid arm form. Figure 15, E, is Platanaster^ a lower Paleozoic 

 asteroid in which the petaloid arm is retained, but the transverse and 

 longitudinal gradients are now equally evident. Next follows Luidia 

 (fig. 15, F), where the gi^adients are still as in Platanaster, but the arm 

 has now lost its petaloid form. On the extreme right (fig. 15, G) is 

 Plutonaster, an astropectinid, in which the transverse gradients are 

 disrupted by longitudinal sliding of the elements, and the base of the 

 arm has expanded by the insertion of new elements, the actinal inter- 

 mediate series. 



Figure 10 presents a corresponding series of cross-sectional views of 

 arms of representative genera, ranging from somasteroids above to 



