283 
of the Ventriculidae of the Chalk. 
and furnishes another characteristic by which it may be at once 
known from V. mammillaris. 
3. Cephalites paradoxus. PI. 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 mdimQ 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. The}^ 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 * 
