CHAP, iv.j THE CRUISES OF THE ' PORCUPINE 163 



pores of the urchins, these were absent in alternate 

 series. Some points about this fossil, particularly 

 the imbricated arrangement of the plates over portions 

 indicating a circle at least four inches in diameter, 

 caused great difficulty in referring it to its place. 

 Edward Forbes examined it, but would not hazard 

 an opinion. The general impression was that it must 

 be the scaly peristome of some large urchin, possibly 

 of a large Cyphosoma, a genus abundant in the same 

 bed. Some years after the discovery of the first 

 specimen, a second was obtained by the Rev. Norman 

 Glass, from Charlton in Kent. This specimen ap- 

 peared at first to solve the difficulty, for it contained 

 in the centre a well-developed e lantern of Aristotle ; ' 

 there then was the peristome of the urchin, of which 

 Mr. Flower's specimen was the periproct. The late 

 Dr. S. P. Woodward examined the two specimens 

 carefully, and found that the question was not so 

 easily settled. He detected the curious reversal of 

 the imbrication of the plates in the ambulacral and 

 interambulacral arese which I have described in 

 Calveria, and at one fpoint he traced the plates over 

 the edge of the specimen, and found that they were 

 repeated inverted on the other side. With great 

 patience and great sagacity he worked the thing out, 

 and came to the conclusion that he was dealing 

 with the representative of a lost family of regular 

 echinids. 



Woodward names his new genus Echinothuria, and 

 describes the chalk species, ~E. floris, almost as fully 

 and accurately as we could describe it now with a full 

 knowledge of its relations for EcUnothuria is 

 closely related to Calveria and Phormosoma. In all 



M 2 



