410 Zoological Society. 
on one edge, and spiniferous or only with a slight tubercle on the oppo- 
site one ; sometimes one edge has two spines and the other only one, 
or a tubercle, and the extent of the truncation of the base differs in 
every example. 
The same examination has also induced me to believe that the spe- 
cimen which these authors have described under the name of Placo- 
trochus levis, p. 283. t. 8. f. 15, is only a variety of the same species ; 
and that Acanthocyathus Grayii, 293. t. 9. f. 2, is only a specimen of 
the same species which has lost its compressed form. I have not 
seen Rhizotrochus typus, p. 282. t. 8. f. 16, or Blastotrochus nutriz, 
p. 284. t. 8. f. 14; but from the figures, I have great suspicions that 
they are only modifications of the same species. 
To give some idea of the variations produced by local causes in 
corals, I may state that the specimens which Messrs. Milne-Edwards 
and Haime have described under the generic name of Heterocyathus, 
are only specimens of the genus Cyathus which have been changed 
in form from their having grown attached to a spiral shell which was 
inhabited by parasitic crustacea. I have specimens showing all the 
grades of change, from the nearly normal conical form of the genus 
to the truncated form which has been described as the type of the 
genus Heterocyathus. This form was well-described by Spengler in 
‘Nova Acta Hafnize,’ 1. 240, and noticed by Gmelin under the name 
of Madrepora Cochlea, p. 3763. 
Messrs. Milne-Edwards and Haime described two species of this 
genus under the names of H. e@quicostatus, t. 10. f. 8, and H. Rous- 
seanus, t. 10. f. 9. Of the former he appears only to have seen a 
single specimen. We have in the British Museum three very distinct 
species, which may be thus described :— 
1. H. Cocutrea = Mad. Cochlea, Gmelin, 8. N. H. equicostatus, 
Milne-Edwards and Haime, 324. t. 10. f. 8. 
Coral subcylindric, hard, white, with narrow, equidistant, distinct 
grooves, crenulated on the edges; base rather dilated ; laminze nar- 
row, sharp-edged, very unequal, grooved on each side, and with 
crowded columns in the centre of the star. 
Hab. Chinese Seas. 
The holes on the outer surface are large and distinct. 
2. H. HEMISPHAERICA. 
Coral subcircular, depressed, subhemispherical, nearly flat below, 
regularly convex above; sides rounded ; plates of star broad-topped, 
as if truncated, covered on top and sides with very numerous crowded 
spines and tubercles ; centre of star roundish, with small columella. 
Hab. Chinese Seas. 
The plates of this species resemble those figured as belonging to 
HI. Rousseanus, 1. c. 325. t. 10. f.9; but the shape of all the two 
specimens in the Museum, which are nearly similar, is quite distinct 
from the view of the side of that species. 
3. H. EUPSAMMIDES. 
Coral polymorphous, base flat, sides shelving, sinuous, surface 
covered with very close, irregular, sinuous, denticulated ridges, and 
