287 
of the Ventriculidse of the Chalk. 
each place, these regular marks are left, contrasting strongly with 
the portions of the membrane adhering to the matrix elsewhere*. 
Fig. 16, which should be compared with fig. 14 of PI. XIII., 
shows the peculiar elevations on the plaits : and fig. 15 is a trans- 
verse section showing three plaits ; the uppermost being struck 
at a point where there is not any projection, the two others just 
at the bend of two projections. 
The specimens which I have distinguished as a variety, under 
the name of annulatus, appear to be cases in which the horse-shoe 
elevations have become more than usually continuous both on the 
same plait and by anastomosis with those on adjoining plaits. 
This character is sometimes seen on the lower part of specimens 
the upper part of which exhibits the true normal characters of 
C. catenifer, as in fig. 9. PI. XIV. In some cases, however, the 
same appearance of connected rings, instead of rows of open 
links, covers a large part, or the whole, of the surface ; and it is 
important that the true place of such specimens should be un- 
derstood, whence the utility of distinguishing them as a variety of 
C. catenifer. 
It generally happens that, even in the most characteristic spe- 
cimens of this variety, there are places in which the projection 
on the outer plait stands, as it so often does in the normal C. ca- 
tenifer, single and wholly unconnected with any other projection 
on the same or on any adjoining plait. In that case, instead of 
being horse-shoe shaped, the circle is usually complete. We thus 
find, on an external plait, a fold very similar to that which cha- 
racterizes the inner plaits of C. retrusus. 
8. Cephalites compressus. PI. XIV. fig. 10. 
Plaits broad and very deep : outer plaits projecting prominently 
in very elongated loops often linked at one extremity and en- 
larged at the other : inner plaits often inclining towards, and 
anastomosing with, adjoining plaits : pouch very short : pro- 
cesses very conspicuous : wall very thick. 
This appears to be quite a distinct species from the last. Its 
fold is looser, approaching therein to the character of the group 
Dilatati. The external modification of fold is very different 
from that of C. catenifer; while the internal difference is even 
more marked. Instead of depressions we have here anastomosis 
with the adjoining plaits; and at the places of anastomosis 
the figure becomes almost angular, instead of circular as here- 
tofore. 
See ante, p. 209, 7iote f- 
