of the Ventriculidse of the Chalk. 



219 



This species unites the characters observed in V. latiplicatus 

 and V. decurrens. 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 between 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 : both 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 modifi^cation 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. 



