EVOLUTION OF THE ECHINODERMS — FELL 469 



transverse (or obliquely transverse) gradients are intersected by 

 subsidiary longitudinal gradients, shown by dotted lines, and 

 swinging outward near the base to produce the petaloid outline 

 of the arm. This pattern, incidentally, is also seen in crinoids. 

 No patterns of this type, in fact none of the patterns illustrated, 

 are found in any other echinoderms. On the evidence so far, it 

 appears likely that the left-hand pattern is the oldest, that the middle 

 pattern arose from it, and that the right-hand pattern arose from 

 the middle pattern. Independent evidence, which space excludes 

 from citation here, shows that the soft structures of the arm indicate 

 a similar sequence. 



To refresh your minds as to the gradients in somasteroids, glance 

 back at figure 5 and note how the gradient field is perpetuated in 

 the adult skeleton, which preserves the gradient patterns under 

 which it must have been produced during development. 



A SURVIVING SOMASTEROID 



Now, having isolated Luidiidae as the most archaic surviving 

 type of asteroid, the obvious next step was to examine each of the 

 surviving representatives of that family. One of the genera dubi- 

 ously referred to the family is Platasterias^ recorded from west Mex- 

 ico 91 years ago and about which little is laiown. 



Figure 8, A, shows the original lithograph published by Gray in 

 1871. Some features immediately arrest our attention — the petal- 

 shaped arms (unknown in any other living asterozoan), the evi- 

 dent flattening of the animal, and what appears to be a transverse 

 arrangement of the underlying internal skeleton. Notice that one of 

 arms is represented by a regenerating stump. The authorities of the 

 British Museum, in which the type specimen was deposited, gener- 

 ously allowed me to dissect this regenerating arm as soon as they 

 received a statement setting out the grounds for suspecting it might 

 throw important light on the affinities of Chinianasteridae. 



Plate 1 shows this same arm, with the regenerating tip, in oblique 

 ventral aspect, after some superficial dissection. Of course, it is 

 at once apparent that Platasterias is not only a somasteroid, but one 

 more closely related to the Chinianasteridae than is ArchegoTiaster. 

 Apart from the characteristic transverse rows of virgalia, it has the am- 

 bulacra! plates in a recumbent position, with a long lateral wing 

 (labeled 1), resting nearly horizontally along the first virgalium 

 (labeled 2) — note that in this aspect the arm is upside down. Between 

 the transverse rows of virgalia are the very grooves which Spen- 

 cer had inferred in his fossils, leading into the narrow and shallow 

 radial groove (numbered 3), below the ambulacral ossicles, and 

 leading to the mouth. A preliminary account of this specimen 

 has since been published (Fell, 1962a) . 



