171 
forms in the series including Belemnites, between this 
bone and Nautilus. The first, Beloptera sepioidea*, ge- 
nerally known as a fossil Sepia, has septa half closing 
an expanded cavity. The second, B. belemnitoidea f, has 
septa quite across a conical cavity : but it is not proved 
that either of these was wholly internal, while it is 
known that the shell of Spirula is partly external, and 
the shell of Nautilus almost wholly so. Now it appears 
very probable that the Belemnite was inclosed, not en- 
tirely within its animal, but within two folds or lobes of 
its mantle or skin, partly, as supposed by Mr. Miller ; 
these lobes, however, might be similar to what would be 
produced by making an incision along the back of a Se- 
pia so as to let the apex of its bone protrude, and so 
bear an analogy to the two lobes that inclose the spi- 
rula, and not be produced by curving the edges or wings 
(fins) of the animal around it, as described by Mr. Miller. 
The idea of the shell having undergone any such an 
alteration by mineralization as to produce the fibrous 
structure, is not consistent either with Mr. Miller’s expe- 
riments or the general characters of changed fossil shells, 
wherein the internal tissue is generally wholly destroy- 
ed, as in Echinus’ spines or siliceous casts, and even in 
parts of some Belemnites. 
Lamarck and all modern authors, except M. de Blain- 
ville, yielding to old prejudices, have continued to de- 
scribe the Belemnite as composed of two parts ; the etui, 
* Sepia Cuvieri. D'Orbigny , Cephalopodes , p. 66. 
f Sepia parisiensis, ib . — A third species of this genus without 
lateral projections has been found at Highgate, and forms an 
additional link. (See tab. 591.) 
