330 THE MEDALS OF CREATION.  Cmap. IX. 
a kind of furrow. The odd ambulacrum is in a deep groove. 
The mouth is transverse, fig. 3, d; and at the anterior part 
of the inferior face there is a depression, which results from 
the convergence of the ambulacral arez towards that point. 
The vent is in the posterior face. This species is from the 
Neocomian strata of France ; I introduce it to illustrate the 
characters of several other echinites, which the French geolo- 
gists suppose to be confined to the so-called Neocomian 
formation ; but which also occur in the Upper Greensand 
of Blackdown. 
Ho.aster is another genus of Spatangide established by 
M. Agassiz, for those echinites that are heart-shaped, with 
simple ambulacra converging towards the summit. The 
mouth is elongated transversely ; the outlet is on the poste- 
rior face. A specimen first described in my Foss. S. D. (pl. 
xvi. jig. 9, 21), as Spatangus planus, is common in the 
Lower Chalk, and Chalk-marl, and abundant in the Firestone 
Malm-rock, 
Our limits will not allow of a more extended notice of 
the fossil Hchinide. The student should consult the Me- 
moirs on the genera, now in course of publication at the 
Government School of Mines, by Professor Edward Forbes; 
the plates are exquisite, as works of art, and the descrip- 
tions all that can be desired. 
Mr. Dixon’s work contains three excellent plates of creta- 
ceous Echinites. Several chalk species are figured in my Foss. 
South Down. The numerous coloured figures of fossil sea- 
urchins in the Pictorial Atlas of Organic Remains, have 
already been mentioned. 
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ye al 
GroLocicaL DisrRipuTion oF Ecutnites.—No vestiges of — 
this order of radiata have been discovered in the Silurian ~ 
deposits : the earliest known occurrence of any type is in the 
tena ines a> Tote sr Ve 
