[ 40 ] 
but whofe origin and formation have never yet been 
fully explained. I lhall not enter into a minute detail 
of the leveral fpecies of the Belemnite. The hiftory 
of this extraneous foffil, or an attempt to account for 
the origin and formation of the Belemnite, fo far as 
they can be difcovered and confirmed by reafonings 
drawn from fads and experience, is the objed of 
the prefent enquiry. I fhall therefore confine my 
fie If to two fpecies of the Belemnite ; the one com- 
mon in moft counties of this kingdom, and vulgarly 
known by the name of Thunder-bolt [a] : the other 
that of the fuiiform or Spindlekind [Z>], found in 
fiate-flone at Stons-field, but in far greater plenty 
in the day near Piddington [c] Oxfordfhire ; anti 
in the chalk-pits of Kent and Surrey [</]. Thofe in 
chalk have been often miftaken for ipines of the 
lea-hedgehog, or Echinus Ovarius ; but the charac- 
terifiics of thele two bodies are widely different. 
The Belemnite breaks in a diredion perpendicular 
to its axis j/]: the fpine obliquely \f]. The Be- 
lemnite, when broken, exhibits central rays j the Spine 
a fmooth refplendent furface. This difiindion is 
. invariable, if the trial be repeated a thoufand times. 
Thefe different appearances are probably the effeds 
of different formations : and therefore the Belemnite 
fieems to be formed by appofition, and the Aculeus 
or Spine by protrufion, or, as Mr. Reaumur calls it, 
by intus-fufception. The radii in the Belemnite are 
owing to the fine laminae, of which it is compofed ; 
they are fo very thin, and break fo nearly alike, that 
they have ever an horizontal furface when broken, 
[rt] T An. III. Fig. i. 
M fig* 4 * M Fig. 
M fig* 2* M Fig* 3 - 
5. [/'] Fig. 6. 
which 
