219 
of the Ventriculidse of the Chalk. 
This species unites the characters observed in V. latiplicatus 
and V. deaurrens. In the former mere points of connection exist, 
on one surface, between the plaits ; in the latter the remarkable 
depressions, bulgings and openings into each other last described 
exist, also on one surface only. In the present species the latter 
characters are found on the inside, the depressions being, how- 
ever, deeper than in V. decurrens ; while, the plaits being deep, 
they exhibit, on the outside, and at distant intervals, points of 
mere connection (no depressions being present on that surface) 
as in V. latiplicatus. 
The difference betw^een the constrictions which mark the 
outer plaits of V. decurrens, and the points of connection, with- 
out any constriction, which exist between the outer plaits of the 
present species, is alone sufficient to distinguish the two. In 
addition to this, however, the present species is much thicker 
and more massive than V. decurrens ; it often attains a thickness 
in the wall of the pouch nearly as great as V. latiplicatus. The 
regularity in the direction of the plaits is another distinguishing 
character. 
Owing to the regularity of the plaits in this species, the figure 
assumed by the inner folds is very regularly quincuncial. 
The inquirer must be careful not to be misled by inspection 
of an inner surface only into determination of this species, as 
such surface in either V. cavatus or V. bicomplicatus, especially 
the latter, and sometimes in V. decurrens, will be difficult to di- 
stinguish. It cannot be too often insisted on that parts of both 
surfaces, as well as sections, should be examined before determi- 
ning species with certainty. 
This species is very local in its distribution, being very rare in 
many places. 
5. Ventriculites bicomplicatus. 
Plaits broad and deep : botb outer and inner plaits deeply de- 
pressed at short and equal intervals, which alternate in ad- 
joining inner and outer plaits ; bulging on each side around 
depressions till adjoining plaits open into each other : pro- 
cesses very conspicuous : wall very thick. 
In the present species we meet 
with a much greater complexity of 
fold than in any preceding one. 
The plaits on both surfaces un- 
dergo a marked modification which 
resembles the modification seen on 
the inner plaits of V. radiatus. The 
form of the depressions on the 
external surface are, however, ge- 
nerally oval instead of round, as in fig. E. 
Fig. E. 
