10 
each surrounded by a depressed areola and a circle of granules. 
Some of the tubercles shew a ligamental cavity in the centre. 
The want of the 4th oviduct may be only an individual pe- 
culiarity, as some recent species are known of which individuals 
have 2, 3, and 4 genital pores respectively; and in other respects 
the characters of the specimen agree with those of the genus 
Rhyncopygus, except as to the position of the anal opening, 
which in the other species of that genus is supra-marginal. 
With respect to this character, as soon as the perfectly radiate 
type in which the opening is in the apex is departed from, it is 
merely a question of degree, whether the opening be higher or 
lower in the posterior interambulacral space, and no connexion 
is known to exist between the position of the opening, when 
excentric, whether marginal or extra-marginal, with any im- 
portant modification of other parts. ‘The margin is not defined 
by any anatomical characters, and moreover in some tolerably 
well defined genera the anal opening is found to vary in position 
in different species. Now we ought not to take as a generic 
character any external peculiarity which is not either known 
to be connected with some important modification of the ana- 
tomy of the creature, or which does not belong. to some feature 
which in some well-defined genera is so constant as to lead us to 
suppose that the law of the creature’s development will not per- 
mit that feature to vary without an otherwise important change 
in the anatomy of the animal; the position of the anal opening 
with reference to the margin satisfies neither of these conditions, 
and for these reasons Prof. Liveing considers the marginal posi- 
tion of the anal opening an insufficient reason for separating the 
specimen from the genus Rhyncopygus. The other species of 
this genus named in Desor’s Synopsis are cretaceous, except 
one from the quaternary deposits of Guadaloupe, so that the 
occurrence of this genus in the crag is a link rather with the 
older than the more recent formations. 
