354 Mr. Toulmin Smith on the Classification 
cretaceous series. I believe only one^ B. digit atus^, is strictly 
common to all the beds, and that undergoes a modification of 
character in the lower ones. Some of the most marked forms 
are, as far as present observation has extended, peculiar to the 
lower of these beds. 
§ a, Operti. 
Brachial folds closed at extremity. 
1. Brachiolites tuherosus. PI. XV. fig. 3. 
Membrane having an irregular and generally slight primary fold : 
brachial fold arranged subspirally around a wide central ca- 
vity, and at rather distant intervals, in tuberous sacs, broad and 
flattened at the head, with slight depressions in the middle of 
the head. 
Of this remarkable species I am fortunate in possessing four 
well-marked specimens, all of which were found by myself, though 
in very distant parts of the country, and they are the only spe- 
cimens I have ever seen. The form is striking. Rising from, 
apparently, a very short root, it attains a considerable height, one 
of my specimens being upwards of three inches high. The sacs 
usually project about four lines from the central cylinder, and 
are about four lines wide, though sometimes more, at their 
broadest part. They open by a broad and trumpet-shaped mouth 
into the eentral cavity, which is wide and open at the top. Thus 
the access of sea- water is freely maintained. 
A transverse section gives the accompanying figure, w^hich will 
be at once distinguished from that seen in a similar section (see 
fig. G, p. 289) of Cephalites capitatus or of any other of that 
genus. 
The figure (3. PI. XV.), which is 
from a specimen carefully developed 
from the chalk by means of the needle, 
gives a complete idea of the species. A 
large part is broken away, and thus the 
inside, as well as the outside, is cleared 
out and displayed. Being, however, 
developed by this means, and the spe- 
cimen being one in which the oxide of 
iron very greatly abounds, the primary fold is hardly to be seen. 
A careful comparison of this with other specimens shows that 
that fold was, like that of Ventriculites impressusy irregular ; 
usually slight ; but occasionally deep, at any rate in that part of 
the membrane which forms the central cylinder. 
The remark already made, in describing Cephalites campanu- 
* See Ann. and Mag. Nat. Hist. vol. xx. 1st Ser. p. 337, and ante p. 38. 
Fig. M. 
