160 Prof. E. J. Chapman on the Structural Relations — 
position of the mouth, as usually given, is erroneous. In several. 
species, as in A. parasiticus and A. Kaskaskiensis of Hall 
(‘Geology of Iowa,’ vol. i. part 2, pl. 25), the centre or origin 
of the rays is a simple disk or rounded tubercle—incontestably, 
no mouth: and hence we may fairly assume that, in other species 
also, the mouth must be situated elsewhere. The question then 
arises as to the real nature of the pyramidal orifice. This is 
usually looked upon either as an anal orifice or as an ovarian 
aperture. Neither of these views is by any means certain, nor, 
indeed, apparently susceptible of proof. To consider this orifice 
as the mouth, however, appears a still less satisfactory conclusion. 
In the Crinoids proper, the true position of the mouth is still, 
strictly speaking, unknown. It is considered in some genera to be 
in the centre of the “ vault,” or upper surface, and in others to 
occupy an excentric position, as between two of the arms, &c. 
This latter view is unsustained by any proof, beyond the mere 
occurrence of an orifice at the points in question. The excentric 
orifice may or may not be the mouth. But if we omit these 
forms from consideration, and turn to those types of Radiata in 
which the position of the mouth is no longer doubtful, that organ, 
it will be seen, is invariably situated in the centre of the body, 
except in the Family of the Spatangide, the highest family or 
natural group of the entire series. In the other families of the 
Kchinida, in the Asterida, Ophiurida, and other orders in which 
the position of the mouth is truly known, the mouth is always 
central. This is evidently its normal position in the radiated type 
of structure, and one, consequently, that we should scarcely 
expect to see departed from, except in the case of those forms 
which stand at the higher limit of the series. Unless this view 
be adopted, we must almost necessarily assume that in the 
Radiata there are certain natural groups (not yet thoroughly 
worked out) which are perfectly unconnected with each other, 
and in which, respectively, the higher forms foreshadow an ad- 
vanced type of structure, whilst the lower forms present the 
normal type. The higher forms of a low group, however lowly 
organized as to their entire structure, will be thus, in certain 
respects, in advance of the lower forms of a higher group. 
Whatever grounds there may be to believe that some law of this 
kind really holds good in Nature, its application in the present 
place would be evidently forced. Discarding, therefore, the idea 
that in the pyramidal orifice of the Cystideans and Agelacrinites 
the mouth is represented, this latter organ must be sought for 
in another place. Reasons have already been stated against this 
being the centre of the rays. Its true position will be found, I 
believe, in the centre of the under side of the body. But, it 
may be urged in objection to this, the genus Agelacrinites is 
