113 



culus, varying much in its form, situation, and size, in different spe- 

 cimens. 



The surfaces of these fossils vary so considerably, as to form very 

 striking differences, on which may be grounded their separation into dif- 

 ferent species. In some their surface is smooth, and in others marked 

 by longitudinal or transverse striae. The parts possessed by the cham- 

 bers are in some marked, externally, by a considerable depression; 

 whilst in others, these corresponding parts are distinguished by a slight 

 degree of protuberance. They differ, also, with respect to their form ; 

 some being straight, while others are described as slightly curved at 

 their smaller end. 



Their size varies, from being, as has been just observed, so small as 

 to be a microscopic object, as to equal the thickness of a man's arm. 

 M. Walch observes, that they are sometimes nearly four inches in dia- 

 meter, and more than an ell long, possessing nearly seventy chambers. 

 A specimen now before me is full four inches in diameter. Dr. Wright 

 describes an orthoceratite in marble, two feet four inches in length, in 

 which were sixty-six partitions. Phil. Trans. Vol. XLIX. p. 670. 



Some care is however necessary, before we determine on the actual 

 form of such of these fossils, as, being imbedded, have had their parts 

 displayed by section. If we suppose a spirulite thus imbedded, and a 

 section commenced at its straight part, should the section not happen to 

 be made on the same plane with that on which the spiral part is dis- 

 posed, a part only of the last turn may be cut through, by which the 

 straight part may be separated from the spiral, and the part of the last 

 turn left with the straight part, might lead those, who did not take this 

 circumstance into consideration, to consider it as a bent orthoceratites. 

 Indeed, so easy it is for mistakes to have been thus made, that I should 

 be disposed to look with suspicion at all the sections of orthoceratites 

 bearing this bent figure, were it not that analogy authorizes the belief 

 that such is sometimes their natural form; since this form is observable 



VOL. III. Q 



