285 
of the Ventriciilidse of the Chalk. 
may easily escape notice. It is however worthy of remark as an 
additional contrivance for gaining extent of surface, and an ad- 
ditional instance of the exhaustless variety of plan which nature 
adopts in the development of life. 
6. Cephalites retrusus. PI. XIV. fig. 8. 
Plaits broad and very deep : outer plaits 
. . . : inner plaits raised in rather small but very promi- 
nent projections at regular and close intervals, and in such 
manner that they range spirally round the whole body, and 
quincuncially relatively to each other : projections cylindrical, 
rounding off slightly at the top and with an exactly central 
and rather deep circular depression (sometimes two) on the 
top of each projection : wall very thick. 
This form departs from every other which has been named. 
It is the first and only instance in which we find projections on 
the inner plaits, which have been already more than once found, 
and will be so again, on the outer plaits. The fold which marked 
the outer plaits of C. bullatus is here found, with striking modi- 
fications however, on the inside. The projections are much 
smaller and closer than in that species, but no less prominent ; 
while each one is again marked by a deep though small and ex- 
actly central depression. It is altogether a very extraordinary 
form*. In chalk specimens it would at once be distinguished 
from every other species by presenting, on its inner surface, the 
appearance of a series of small rings, quite unconnected with each 
other, but arranged with the utmost regularity. 
It is an extremely rare species. I have only met with a single 
specimen, and that is a cast of the inner surface in fiint, with frag- 
ments of the characteristic ventriculitic structure preserved in 
* Forms like this afford very strong ground of caution against the hasty 
adoption of any development theories. The whole of the present subject af- 
fords, indeed, the strongest ground for such caution. We see infinite variety 
— all subservient to the ends of life ; and throughout which one Unity is 
traceable ; but a Unity which certainly no more points to a low type of orga- 
nization, or to a necessary or probable progressive development of one form 
from another, than does the beautiful and philosophical demonstration of the 
cranial vertebrae, or the fact of that demonstration being afforded by the 
most different members of the Vertebrata. It should be noticed that the 
very remarkable octahedral structure already developed as characteristic of 
the membrane of the Ventriculidae has no relation whatever to those “geo- 
metrical figures ” alluded to by Professor Owen. In the present case it 
is a relative, and not a positive, form ; and one assumed by animal fibre for 
a special purpose. It has been already remarked (p. 96) that no spicules, 
or “ calcifying salts ” enter into the composition of any of the Ventriculidae. 
See Owen “on the Archetype and Homologies of the Vertebrate Skele- 
ton,” 1848, p. 171. 
