CHAP. Iv. ] 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 ‘lantern of Aristotle; ’ 
there then was the peristome of the urchin, of which 
Mr. Flower’s specimen was the periproct. The late 
Dr. 8S. P. Woodward examined the two specimens 
carefully, and found that the question was not so 
easily settled. J1e detected the curious reversal of 
the imbrication of the plates in the ambulacral and 
interambulacral ares which I have described in 
Calveria, and at one point 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 Lehinothuria, and 
describes the chalk species, /. floris, almost as fully 
and accurately as we could describe it now with a 
full knowledge of its relations—for Lchinothuria is 
closely related to Calveria and Phormosoma. In all 
M 2 
