27 
This fossil is very nearly circular. Its upper surface is convex. 
The base is flat, with five narrow and slightly excavated grooves, 
extending in right lines, and at nearly equal distances, to the margin. 
The mouth is rather injured, so that its shape cannot be determined. 
The anus is small and round, and is placed at about a fifth of the 
diameter, from the margin, in an area which is rather smaller than 
the others. The ambulacra appear to have borne the figures of oval 
petals ; and are each composed of a line formed of single pores, 
surrounded by three, four, five, or even six lines of minuter pores, 
obliquely disposed in very small grooves. What figure resulted from 
their approximation in the centre cannot be determined, as the shell 
is in that part broken. 
On examining the surface of this fossil with a lens, it was found 
still to retain, in several parts, the small flat imbricating spines. These 
are represented in the sketch on the right side of the fossil. 
Echinodiscus reticulatus is not known fossil. E. Orbicularis, Tab. 
xlv. Fig. 6, 7 ; Lang. Tab. xxxv. Fig. ultim.; is a depressed orbicular 
echinite, about an inch in diameter, with acute oval ambulacra, and 
ten porous rays in the base ; the mouth round and the anus small, 
and midway between the centre and the margin. E. Rosaceus, Tab. 
xn. 4, Lesk. et Tab. e. hi. 8, Knorr. differs from the former in being 
much smaller, and in its ambulacra forming a floweret, with very 
short petals, round the vertex. 
Echinodiscus decies digitatus, octodigitatus et dentatus, do 
not appear to be known as fossils. 
Under the class Catocystus a new genus has been formed, by 
Phelsum, and named Echinocyamus. The generic characters are, 
ten stellated ambulacra, passing from the top in straight biporous 
rows ; the mouth and anus adjoining in the middle part of the base. 
The shells of this genus are not known in a petrified state. 
