215 
of the Ventriculidse of the Chalk. 
which appear so widely different from each other in all that meets 
the eye of the general observer^ the eharaeters which each exhibits 
may be as readily understood as in the present species by taking 
as the basis that simple Unity of plan which has been thus illus- 
trated. 
2. Ventriculites latiplicatus. 
Folds very broad and deep : outer plaits simple and regular : 
inner plaits inelining, at intervals, towards, and anastomosing, 
a little below the surfaee, with, adjoining plaits : wall very 
thick. 
This is a very interesting species. It presents us with the first 
instanee of the plaits departing from their entire individuality, 
and of a connexion being formed between adjoining ones. In the 
present species that connexion is merely by a slight anastomosis 
at distant intervals. There being no depression in the plait, on 
either surface, the access of water is perfectly preserved without 
any additional provision such as we shall find in V. radiatus 
and many others. 
In V. mammillaris the departure from the simple plait took 
plaee on the outside. In the 
present species it takes place on 
the inside, and is more marked 
in its character and degree ; but 
it is none the less an illustration 
of the principle of Unity which 
it has been endeavoured to ex- 
plain, being elearly but a modi- 
fication of the simple plait. 
This will be understood from 
the following figure, in whieh 
h, b, b are the low^er plaits; c, c, Cy 
the upper plaits; and a, a, a, 
the points of anastomosis. We shall, in the next species, find 
the eomplexity of the fold much increased. 
The wall of this species is thicker than that of any other spe- 
cies of the genus Ventriculites. 
3. Ventriculites decurrens. PI. XIII. fig. 8. 
Plaits often irregular in direction : outer plaits constricted at 
distant and unequal intervals : inner plaits depressed at short 
and nearly equal intervals ; bulging on each side around de- 
pressions till adjoining plaits meet and open into each other : 
processes eonspicuous : wall of moderate thickness. 
Var. tenuiplicatus. PI. XIII. fig. 9. 
Plaits close and delicate : , outer plaits nearly regular in direction 
Fig. D. 
