15 
x. a. Fig. 3, 4, is represented a fossil with its cast, which M. Walch 
considers, with M. Genzmer, as an echinus totally different from any 
with which we are acquainted. There are in this fossil no tubercles, 
ambulacra, nor sutures, as in other echini ; but its surface is nearly 
covered with a kind of trelisse work, formed by lines passing in 
almost every direction, so as to form figures bearing somewhat of a 
stellated appearance. A slight sketch, showing the form of the 
surface of this fossil, as given by Walch, is shown Plate I. Fig. 14. 
Plate I. Fig. 12, is an echinite found in the green silicious sand, so 
requent in sev ei al parts of Wiltshire. The figure here given is mag- 
nified to about twice the size of the fossil. Round the superior opening 
of this echinite may be seen a remarkable extension of the appendage 
above mentioned ; formed chiefly in roundish plates, connected to- 
gethei by numerous short filaments, and reaching over nearly a third 
part of the surface. In another specimen, these plates are hexagonal 
and exactly fitted to each other. Plate I. Fig. 13, is another echinite 
from the same part, which is particularly interesting, from its appear- 
ing to be highly illustrative of the fossils above mentioned. The anal 
appendage is here seen, with a trelissed surface, almost exactly similar 
to that of M. Walch’ s fossil, and extending so low down, as to cover 
nearly one half of the echinite. If, indeed, this surface had extended 
over the whole echinite, it would have very closely resembled M. 
Valch s fossil, and have yielded us no further information : but suf- 
cient of the inferior part of the surface of the echinite is left un- 
coverc , to allow us to discover, that it has all the characters of a 
beautiful variation of the C. diadema. 
Still, however, we are without any positive information as to the 
nature and office of this part, which is so singularly organised It is 
on future specimens, and on further observations on the living animal, 
that we must depend for information on this subject. Whether this 
is a permanent appendage or not; whether it belongs to particular 
species only ; or whether it is a part essential to the animal of each 
