218 
Mr. Toulmin Smitli on the Classification 
exhibited numerous shallow depressions ; and which plaits^ on 
each side of these depressions^ were united together by small 
transverse tubules, the mouths of which tubules were directed 
towards the outer' surface of the whole body, and opened, always, 
into the interspaces between the outer plaits. 
It will be obvious that sections, whether made longitudinally 
or transversely to the plaits, will vary in numberless ways in the 
figure which will be seen, according to the particular point of 
the plait through which the section passes. This must be re- 
membered when the section seen at the edge of a flint or frac- 
tured chalk specimen is examined. This observation applies as 
well to the present species as to almost every other which will 
subsequently come under notice. 
The plaits of V. decurrens are not very regular in their course. 
On the contrary, the outer plait, where the course is easily traced, 
is often very winding, though several generally run parallel to 
each other. The constrictions occur at irregular and distant in- 
tervals, and cause the surface to appear as if several of the round 
elevations of V. mammillaris had run together down the surface, 
— a modification which may truly be considered to represent the 
fact. Hence the specific name. The constrictions do not extend 
the whole depth of the plait. They do not appear ever to ex- 
tend so deep as to interfere with the mode of fold eharacteristic 
of the inner surface. It is owing to the winding course of the 
plaits in this species that the rings seen on the inside do not 
assume, as in V. radiatus, a perfectly regular quincuncial figure ; 
the places of the depressions and accompanying characters of 
structure varying, of course, in places, as the plait winds. In 
V. tenuiplicatus, where the plaits are more regular, the quincun- 
cial figure on the inside is correspondingly more regular. The 
constrictions on the outer plaits of that variety are much slighter 
than in V. decurrens. They resemble much more a very slight 
exhibition of the normal eharacters of V. mammillaris. They thus 
rarely interfere with the continuous course of the plait. 
4. Ventriculites radiatus. PI. XIII. figs. 10, 15. 
Plaits broad and deep : outer plaits regular, and with an occa- 
sional lateral eonnection : inner plaits deeply depressed at 
short and equal intervals ; bulging on each side around de- 
pressions till the adjoining plaits meet and open into each 
other : processes very conspicuous : wall thick. 
I have retained for this interesting species the name radiatus 
of Mantell, as being the species which comes nearest in super- 
ficial external and internal appearance to his descriptions already 
cited*. 
* Ante, vol. xx. p. 76. 
