26 
VICTORIA MEMORIAL MUSEUM. BULLETIN NO. I 
is made up of wedge-shaped plates which are longer than wide. 
On the upper part of the specimen, the inner half of each plate, 
or slightly less than half, is solid and tuberculated, while the 
outer half, or a little more, is excavated, so that two spoon- 
shaped depressions are formed. Nearly the whole thickness of 
the shell is excavated, and the outer edge of the solid part of the 
plate is under-cut, as is shown in the side view of 
one of these plates. This under-cutting extends into the solid 
portion of the plate as a cone-shaped pit leading inward from 
each spoon-shaped basin, and there are probably pores through 
the plates, although the evidence on that point is not clear. The 
spoon-shaped basins, two to a plate, continue all around the 
outer side of the ring, and form a circular canal, which is covered 
with small plates. There are two series of these small plates. 
The first series consists of a row of small, curved plates which 
are nearly vertical, and form the outer wall of the canal. These 
plates frequently fall outward on the disintegration of the 
animal, and are then seen as an extra series of plates outside the 
sub-marginals. This condition is shown in the specimen of C. 
huronensis , and in Hail's figure of C. salteri. 
The second series consists of a double row of very small, 
roughly pentagonal roofing plates, their outer ends somewhat 
pointed, and interlocking. They are arranged so that those of 
the inner and outer row alternate in position, and there are about 
4 or 5 to a sub-marginal plate. These plates are very loosely 
attached, resting on the edges of the vertical outer plates on the 
outside, and in the groove formed by the undercut on the inside. 
They were probably movable. These plates were observed by 
Billings, and figured, although inaccurately. 
The large plates of the sub-marginal ring are not set firmly 
against each other, but have a space between, and this space en- 
larges from the middle towards both upper and lower surfaces. 
The sides of the plates are vertically striated, as is shown in 
Billings', Hall's, and Miller’s figures, and, as has already been 
suggested, these striations indicate cartilaginous or muscular 
attachments between the plates, giving considerable flexibility 
to the ring. 
The specimen now being described preserves a few of the plates 
of the lower side, seen from the inside, showing their form and 
loose arrangement, but presents no other detail of the disk. 
