274 O-RTHOCERATITE.- 



-r-Orthoceratite, Lituite, Baculite, Hamite, Scaphite, Turri- 

 lite, Nummulite, Belemnite.* 



Orthoceratite, PL 44, Fig. 4. 



The Orthoceratites (so called from their usual form, — 

 that of a straight horn) began their existence at the same 

 early period with the Nautili, in the seas which deposited 

 the Transition strata ; and are so nearly allied to them in 

 structure, that we may conclude they performed a similar 

 function as floats of Cephalopodous Mollusks. This genus 

 contains many species which abound in the strata of the 

 Transition series, and is one of those which, having been 

 called into existence amongst the earliest inhabitants of our 

 planet, was at an early period also consigned to almost total 

 destruction-! 



An Orthoceratite (see PL 44, Fig. 4) is, like the Nautilus, 

 a multilocular shell, having its chambers separated by trans- 

 verse plates, concave externally, and internally convex ; and 

 pierced, either at the centre or towards tiie margin, by a 

 Siphuncle, (a.) This pipe varies in size, more than that of 

 any other muhilocular shell, viz. from one-tenth to one-half 

 of the diameter of the shell ; and often assumes a tumid 

 form, which would admit of the distension of a membranous 



Peron having been lost, is in some degree removed by Captain King's dis- 

 covery of another of these shells, attached to a fragment of tlie mantle of an 

 animal of unknown species resembling a Sepia, which I have seen in the 

 possession of Mr. Owen, at the Royal College of Surgeons, London, 



* In the genus Lituite, Orthoceratite, and Belemnite, PI. 44, /. 3, 4, 17, 

 the simple curvature of the transverse plates resembles the character of the 

 Nautilus. In the Baculite, Hamite, Scaphite, and Turrilite, PI. 44, Fig. 5' 

 8, 12, 13, 14, 15, the sinuous foldings and foliated edges of tlie transverse 

 plates resemble those of the Ammonites. 



t See D'Orbigny's Tableau Rletiiodique des Ccphalopodos. 



Tiiere arc, I believe, only two exceptions yet known to the general 

 fact, that the genus Orthoceratite became extinct before the deposition 

 of the Secondary strata had commenced. The most recent rocks in 

 which they have been noticed, are a small and problematical species ijs 



