92: PHILOSOPHICAL TRANSACTIONS. [aNNO I764. 



belemnite ; the one common in most counties of this kingdom, and vulgarly 

 known by the name of thunder-bolt (pi. 3, fig. 1) : the other that of the fusi- 

 form or spindlekind, (fig. 2) found in slate-stone at Stons-field, but in far 

 greater plenty in the clay near Piddington (fig. 3) Oxfordshire, and in the 

 chalk-pits of Kent and Surrey, (fig. 4). Those in chalk have been often mis- 

 taken for spines of the sea-hedgehog, or echinus ovarius ; but the characteristics 

 of these two bodies are widely different. The belemnite breaks in a direction 

 perpendicular to its axis, (fig. 5) : the spine obliquely (fig. 6). The belemnite, 

 when broken, exhibits central rays ; the spine a smooth resplendent surface. 

 This distinction is invariable, if the trial be repeated a thousand times. These 

 different appearances are probably the effects of different formations : and there- 

 fore the belemnite seems to be formed by apposition, and the aculeus or spine by 

 protrusion, or, as Mr. Reaumur calls it, by intus-susception. The radii in the 

 belemnite are owing to the fine laminae of which it is composed ; they are so 

 very thin, and break so nearly alike, that they have ever an horizontal surface 

 when broken, which is common to all the shells of the trichite kind.* The 

 spine being formed by protrusion, its component parts are adjusted in a different 

 manner, and the pores, like the cancelli in bones, (though not so distinct) are 

 irregular, which is the reason of its breaking obliquely in any direction, but it 

 is generally smooth by being saturated with a plated kind of spar.-|- 



Mr. Brander, in a dissertation on the belenniite, presented to the r. s.,+ 

 justly observes, " that the belemnite belongs to the testaceous part of the ani- 

 mal kingdom, and to the family of the nautili." And it may be further added, 

 that this gentleman's sentiments are greatly strengthened by the surprising 

 analogy which the belemnite bears to the little pearly concamerated shell, or 

 cornu ammonis ; and the orthoceratites, to the large nautilus ; the former 

 having its siphunculus on the verge, as the latter has it in the centre of the dia- 

 phragm, or partition of each cell or chamber. " It has indeed been truly 

 matter of speculation, continues Mr. Brander, how this huge solid substance 

 called the belemnite, exclusive of the nucleus, could be formed ; and how it 

 happens, that some belemnites should have the nucleus within them, others not; 

 the cavity to contain the same in some very small, in others scarcely or not at all 

 visible." — But Mr. P. thinks it will be found on inquiry, that these are only 

 circumstances which are common to other testaceous bodies, that have been ac- 



• Fig. 7. A piec of the penna marina, perforated by the pholades. — Orig. 

 ' f Spar seems to be nothing but chrystal debased by a calcareous earth : the more debased sort 

 breaks in a hairy trichite manner, the more pellucid kind with a smooth surface ; and always in an 

 oblique rhomboidal direction ; which perhaps may in some measure enable us to account for itg 

 double refraction. — Orig. 



X Philosophical Transactions, vol. xlviii. for 1754, page 803. — Orig. 



