291 Mr. Toalmin Smith on the Ventriculidse of the Chalk. 
The species rarely attained half an inch in height or an inch 
in breadth, though specimens often extend between two and 
* three inches in length. 
It seems to me that the cephalic constrictions most probably 
mark periods of growth *. They vary in number much in dif- 
ferent specimens, and, as has been seen, are sometimes not found 
at all, in which case there are several openings in the undivided 
head. 
Specimens sometimes assume irregular forms, as if, after 
death, the long body had become twisted, which I have little 
doubt was, in many such cases, the real fact. 
I have placed this species next in order to C. campanulatus^ 
inasmuch as, on the one hand, the mode of attachment of the 
cephalic membrane to the plaits resembles very much that which 
is found in C. campanulatus^ while, on the other hand, the fact of 
the openings in the head of this species being generally several 
instead of only one, places it in some relation to the species which 
will next claim attention. 
4. Cephalites perf or atus. PI. XV. fig. 2. 
Plaits wide and very deep, so as to leave no distinct and single 
central cavity ; dividing, and so reduplicating, very constantly, 
longitudinally, but not transversely ; somewhat winding both 
longitudinally and laterally ; occasional points of anastomosis 
near the outer surface : head covering the entire top and round- 
ing to some distance down the sides ; having several small 
round perforations arranged without any regular figure ; body 
of nearly uniform breadth and often twice the height of its 
diameter. 
The peculiar arrangement of the plaits and head in the last 
two species rendered any anastomosis of adjoining plaits not 
essential in either of them. The much greater height of the 
present species rendered occasional points of anastomosis an im- 
portant means of securing the permanence of the position of the 
folds. The width and depth of those folds rendered a large 
head necessary, while it made unnecessary any large single cen- 
tral cavity ; the several small openings in the head giving suffi- 
cient access to the sea- water for the purpose of bathing freely all 
the internal surface of the polypiferous membrane. The unity 
of formt is not in the least degree impaired by the existence of 
these several points of access. The one head still holds in place 
all the several plaits, a contrivance for the security of the entire 
* Specimens, apparently entire, are sometimes found, having one only 
of the rounded divisions, and thus bearing some resemblance to a very small 
C. campanulatus, with its root at one end instead of at the base, 
f See before, p. 207 note. 
