21S Mr. Toulmin Smith on the Classification 



exhibited numerous shallow depressions; and which ])laits, on 

 each side of these depressions, were iinited together by small 

 transverse tubules, the mouths of which tubules were directed 

 towards the oute?' 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 characteristic 

 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 wands. 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 characters 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 connection : 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. 



