66 



which appear so widely different from each other in all that meets 

 the eye of the general observer, the characters 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. 

 Plaits very broad and deep : outer plaits simple and regular : 



inner plaits inclining, at intervals, towards, and anastomosing, 



a little below the surface, with, adjoining plaits : wall very 



thick. 



This is a very interesting species. It presents us with the first 

 instance 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 

 place 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 clearly but a modi- 

 fication of the simple plait. 

 This will be understood from 

 the following figure, in which 

 b, b, b are the lower plaits; c, c, c y 

 the upper plaits; and a t a, a, 

 the points of anastomosis. We shall, in the next species, find 

 the complexity 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 conspicuous : wall of moderate thickness. 



Var. tenuiplicatus. PL XIII. fig. 9. 

 Plaits close and delicate : outer plaits nearly regular in direction 



