1856.] WOODWARD ORTHOCERAS. 379 



these : — I. When buried in the sea-bed, mud entered the siphuncle 

 and filled the remains of those chambers in which the siphuncle was 

 incomplete. 2. Water containing carbonate of lime in solution 

 penetrated the air-chambers and other closed spaces, and coated all 

 the surfaces with tufa. 3. The shell was dissolved and removed, 

 before the consolidation of the surrounding mud, which thus obtained 

 access to all those cavities whose calcareous lining was incomplete. 

 4. The cavities which the mud could not enter were filled, or nearly- 

 filled, with crystalline carbonate of lime. 



The same structure is exhibited, with great regularity, in the 

 small specimen represented by fig. 2 (the locality of which is un- 

 known) ; in this the more highly curved lines alternating with the 

 septa represent the collapsed lining of the air-cells. 



I have before noticed similar appearances in many polished sections 

 of Actinoceras, especially those from the black limestones of New 

 York, one of which is represented in PI. VI. fig. 3. In these "it is 

 evident that the mud has gained access to the air-chambers along 

 the course of the blood-vessels ; but the chambers are not entirely 

 filled, because their lining membrane has contracted, leaving a space 

 between itself and certain portions of the walls, which correspond in 

 each chamber *. 



The collection of Prof. Tennant contains the apex of a small Or- 

 thoceras from the Carboniferous Limestone of Ireland, one side of 

 which is fractured, showing what appears to be an enormous siphun- 

 cle, slightly moniliform, and nearly filling the shell (fig. Ad). On 

 making a section of this specimen, however, the true siphuncle 

 (fig. 4 5, s) proved to be small, central, and cylindrical, and contracted 

 at each septum. The lining membrane of the air-cells has separated 

 from the outer shell-wall only (c, c), producing the appearance 

 noticed on the outside. 



A specimen of the same species of Orthoceras, in the British 

 Museum, measures a yard in length and 6 inches in diameter at the 

 larger end, although the body-chamber is nearly all wanting. Part 

 of the apex has been slit, and shows the same structure as Mr. Ten- 

 nant's specimen, but is less regular, and the septa are closer. 



Something of this kind was noticed by Mr. Charles Stokes in a 

 Russian Orthoceras (Geol. Trans. 2nd series, vol. v. p. 712. pi. 60. 

 f. 4), and was attributed to a separation of the laminse of the septa. 

 The figure is very obscure, and the specimen probably lost f. 



It will probably be found that these appearances are of constant 

 occurrence in the shells of this genus ; and that they arise from 

 changes which took place in the lifetime of the animal, commencing 

 at the apex and progressing onwards, and resulting to a greater or 

 less extent in the death of the shell. 



* Manual of the Mollusca, 1851, p. 82. 



t At the sale of Mr. Stokes' collection all the most important specimens of 

 Orthocerata were purchased for the British Museum ; but unfortunately many of 

 those figured in the 'Geol. Trans.' by Dr. Bigsby (2nd series, vol. i.), including 

 the type of Bronn's genus Conoceras, and others ^figured by Mr. Stokes himself, 

 could not be found. 



VOL. XII. — PART I. 2d 



