211 
of the Ventriculidse of the Chalk, 
cuncialis is smooth. It will be perceived that where, as in fig. 12, 
the folds are broad on each surface at their margin and narrow 
to a point at their base, the effect must be to give to the whole 
wall of the pouch a rough surface. The wall is usually not very 
thick. I have never seen it attain so great a thickness as V. 
quincuncialis often does. The height and depth of the folds 
render it difficult ever to confound it with V. impressus, in which 
also the same smooth general appearance of the surface as in 
V. quincuncialis is usually maintained, and for the same reason 
as in that species. 
5. Ventriculites tessellatus. PI. Xlll. figs. 2, 3, 4. 
Membrane folded in regular quadrilateral and rectangular figures 
usually more or less oblong : wall of moderate thickness. 
In Mr. Morrises Catalogue mention is made of a Ventriculites 
quadratus, and reference is made to Goldfuss, pi. 33. fig. 1. I 
have already* stated that this is no Ventriculite. The description 
is, seriebus pororum oblongorum rectis parallelis decussantibus” 
— an error into which I cannot conceive it possible that Goldfuss 
could have fallen if he really had the present species before him. 
And the magnified sketch given in fig. 1 of the same plate, — 
unless fancy and not actual observation be there copied, — differs 
as widely from any characters of the present species, and from 
any form of Ventriculitic structure, as does the description which 
has been cited. 
This form is rare ; and it is still rarer to find a perfect speci- 
men. I have several fragments, excellently well displaying struc- 
ture and fold, and during the summer of 1847 I was fortunate 
enough to find a perfect specimen, with roots and margin, and 
both body and cast, entirely perfect, and five inches in diameter ; 
but I was so unfortunate as for it to break in pieces before I could 
convey it home, a danger to which the collector of these delicate 
fossils is peculiarly liable. Though this loss is not easily to be 
replaced, the opportunity of verifying the entire form of the spe- 
cies was important. The rarity of the species has prevented me 
from making sufficiently extended observations on the moveable 
processes to state, with confidence, anything definite in regard to 
them. 
In this species the plaits may often readily be traced, with the 
oblong depressions running along them on each face of the wall. 
This being borne in mind, the figures often displayed on section 
will readily be understood, when, instead of the regular alter- 
nation of depressions seen at one place of section, as in fig. 4 
Ante, vol. xx. p. 78. 
14 * 
