710 PEOP. W. J. SOLLAS ON SILURIAN [NoV. 1 899, 



Protocidaris appears, and it is quite possible that both Mr. Whid- 

 borne's specimen and that in the Oxford University Museum may 

 represent an unusually large form of Echinocystis. 



The general question of the evolution of the Echinoids 

 is greatly elucidated by a study of the Palceodiscus and Echinocystis 

 just described. The comparatively early appearance of the Asteroids, 

 as shown by the occurrence of Palceasterina in the Tremadoc Slates 

 of St. David's, and the marked Asteroid affinities of the Silurian 

 Echinoids, particularly of Palceodiscus^ suggest that the Echinoid 

 branch has been directly derived from the Asteroid, rather than 

 that both have descended from a common stock. 



Adopting the former as the more probable hypothesis, it has 

 already been shown that the ambulacral plates of the Echinoids 

 may be regarded as the survivors of what was once a twofold 

 series, and an attempt may next be made to find an interpretation 

 of that complex buccal armature, ' Aristotle's lantern.' This is by 

 no means an easy task. Dr. Gregory has taken a first step, in 

 conjecturing that the pyramids may have been produced by a 

 modification of ambulacral plates; and if this be not precisely 

 true, it is at least suggestive. 



It may be observed at the outset, that the pyramids of the 

 lantern lie, like other plates of the true Echinoid test, superficial 

 to the water-vascular system, and hence must be correlated with 

 the outer and not the inner ambulacral series of Palceodiscus and 

 its allies. But the buccal armature evidently arose very early in 

 the development of the phylum, possibly even determined its origin, 

 and consequently the outer ambulacral series, from which we 

 imagine the lantern to have been mainly derived, must have also 

 made its appearance at a very primitive stage. Neumayr, who 

 pointed out the significance of the structure of the ambulacra in 

 Mesites, and suggested an explanation of the ambulacral system of 

 the Echinoids similar to ours, asserted that a superficial series of 

 plates occurs outside the radial nerves and water-canals in young 

 Asteroids, and subsequently disappears. If so, the twofold series 

 may be regarded as originally common to the two groups : the 

 outer, becoming functionless with the arching of the ambulacra, was 

 lost in star-fishes ; while the inner, becoming functionless as the test 

 acquired rigidity, was lost in the Echinoids. 



A comparison of the buccal armature in star-fishes and urchins 

 reveals a certain general resemblance. In the former a pair of 

 jaws, recalling the alveoli of Echinoids, are braced together by the 

 * odontophore ' in much the same way as the pyramids of the 

 lantern are united by the rotula : which, it may be added, lies 

 deeper than the water-canal, and thus recalls an Asteroid relation- 

 ship. The jaws of the star-fish bear spines; the alveoli of the 

 urchin carry each a tooth. The odontophore occupies an inter- 

 radial, the rotula a radial position ; and the surfaces of muscular 

 attachment of the jaws or pyramids (strangely similar in the two 



