of the Ventriculidse of the Chalk. 283 



and furnishes another characteristic by which it may be at once 

 known from V. mammillaris. 



3. Cephalites paradoxus. PL XIV. fig. 3. 



Plaits narrow but deep : outer plaits depressed irregularly ; 

 bulging around depressions till the adjoining plaits meet and 

 open into each other : inner plaits regular and simple : pro- 

 cesses conspicuous : wall thick. 



I have given to this remarkable species the name paradoxus, 

 because it differs from every other species of this genus in having 

 the plaits simple and regular on the inside, while all the com- 

 plexity is on the outside. 



The depressions, and consequently the interspaces between the 

 anastomosing bulgings on the outside, are not of a regular figure, 

 as is the case on the inner surfaces of Ventriculites radiatus and 

 other species. They are varying and elongated; often almost 

 angular ; though, as the plaits are narrow, never very large. There 

 do not appear to be any points of anastomosis* between the ad- 

 joining inner and regular plaits, such as are found between the 

 outer plaits of V. radiatus. 



4. Cephalites alternans. PI. VII. (vol. xx.) fig. 2, & PI. XIV. 

 figs. 4 & 5. 



Plaits rather broad and very deep : both outer and inner plaits 

 depressed at unequal intervals ; bulging on each side around 

 depressions till the adjoining plaits meet and open into each 

 other : processes conspicuous : wall thick. 



The mode of fold in this species resembles that of Ventriculites 

 bicomplicatus in the fact of being repeated on the plaits of each 

 surface. It differs essentially, however, in the fact that the de- 

 pressions, though generally round, are, on neither surface, at re- 

 gular intervals : consequently no regular figure is assumed in the 

 general aspect of either surface. 



I have named the species alternans from the circumstance of 

 the repetition on the two surfaces of the same manner of fold ; 

 while the straight plait is clearly traceable in the central portion 

 of the wall. A transverse section of a specimen of this species is 

 seen on PI. VII. fig. 2. Its difference from a similar section of 

 C. longitudinalis is very marked. 



This is an extremely rare species. 



* The appearances seen on dissecting away the inner surface must not 

 be mistaken for this anastomosis. They are, in fact, the bases of the de- 

 pressions on the outer plaits. See the description of a similar appearance 

 on the outside of V. tenuiplicatus, p. 217. 



19* 



