294 Mr. Toulmin 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 perforatus. 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 formf 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. 



