360 Mr. Toulmin Smith on the Classification 
squares being smaller than in any other species of the Ventri- 
culidje. Its membrane is folded up in very small depressions, 
after the manner of Ventriculites quincancialisy but very much 
smaller, each depression being rarely more than the sixth of a line 
in diameter, often less, and usually of an oval form. The plaits 
may generally be easily traced, as, though these folds assume a 
quincuncial arrangement, it is usually easy, on a large surface, 
to trace the fan-like expansion of certain lines, — the lines of the 
typical plait. The squares being smaller than in other species 
of Ventriculidse, the whole thickness of the folded membrane does 
not exceed the twentieth of an inch. The two surfaces corre- 
spond as in the section Simplices of the genus Ventriculites, 
Rising from a short and, comparatively to the whole size of 
the animal, small central root, the fold is sometimes found for a 
short distance assuming a tubular form, but it generally puts on, 
very speedily, its characteristic brachial fold into narrow flat tri- 
angular arms. These folds vary greatly in number. The spe- 
cimen above figured has ten arms, but that number is very un- 
common. I have even specimens with only two, but I consider 
them as abnormal. The arms do not always, though generally, 
start from exactly the same point. This is the case with the arm 
on the right of the above figure, as well as with some others the 
bases of which cannot be seen in the present position of that 
figure. 
Spreading out, sometimes to a very great, sometimes to a very 
small size, the angular fold terminates, at the top, not in double 
parallel edges as might have been anticipated, but with the edges 
united and rounded over as at the lateral margin. Each arm 
is thus closed on all sides except where it communicates with the 
central cavity, which is, normally, open at the top. The upper 
and lateral margins of each arm consequently form with each 
other an acute angle. Those margins are usually straight ; those 
at the top being also often horizontal, and those at the sides 
sloping regularly down from their extremity to a point at the 
base. Instances are found, however, in which these margins 
have a symmetrical wavy outline. At the extreme outer angle 
of each arm there is fui’ther added a curious triangular expan- 
sion, not unlike the expansion at the extremity of each sac of 
B. tuberosus. This lip, as it may be termed, and which, like all 
the elevations on Ventriculites mammillariSy B. tuberosus^ &c., is 
hollow within, is concave at the aspect towards the spectator. 
It will be understood from this that, between the two walls of 
each lobe, there is an open space, narrow and of even width 
throughout. The communication between the central cavity and 
these arms being therefore not wide, though continuous, the ex- 
tent and narrowness of these arms struck me for a long time as 
