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nearly three quarters of an inch, cavity one inch and 
three quarters deep. 
We have ventured to arrange under one specific ap- 
pellation, Mr. Miller’s Belemnites electrinus and Acti- 
nocamax verus, and almost all the species of the second 
and third of M. de Blainville’s sections of the genus. 
The B. plenus, which forms the first section, and which 
M. de Blainville confounds with Actinocamax verus, is 
a distinct species, rather fusiform, gradually tapering to 
a point, and not mucronated nor marked with what ap- 
pear like the impressions of veins upon its surface : we 
have not seen a specimen of which we know the locality, 
but possess two very good ones that appear to have been 
imbedded in clay. The form of the base, which has 
led Mr. Miller to separate the Actinocamax, and M. de 
Blainville the two first sections of the genus from one 
species, is to be explained as follows. When the shell, 
upon the death of the animal, sunk from its native ele- 
ment into the mud, the thin sides of its conical cavity 
were soon worn away ; the two surfaces being both ex- 
posed and worn, the cavity was gradually shortened, as 
we find it in Blainville’s second section, where in some 
cases a portion of the fissure still remains, the same ac- 
tion continuing until the cavity had entirely disappeared, 
one surface alone remained to be acted upon, which was 
rendered first flat and thin, in consequence of the edges 
being most worn, then convex or conical as in the Acti- 
nocamax. Small individuals are most subject to this com- 
plete metamorphosis, because in them it would much 
sooner take place under the same circumstances. The base 
is either oval or triangular, or perhaps even squarish, 
according to the part of the tube of which it is the 
section, and the depth of the vein-like impressions that 
flatten the sides. No two specimens are found precisely 
similar in the depth of the worn cavity nor the convexity 
of the base, and all are more or less irregularly striated 
and furrowed from the centre. 
We have no proof of the existence of septa in the ca- 
vity. M. Blainville has asserted that they do not exist ; 
and they are not described by Mr. Miller, although he 
