Mr. Stokes on Orthocerata. 711 



If, then, these Huroniae are to be considered as the siphuncles of Orthoce- 

 rata, (and such is the conclusion I have adopted from the evidence just stated,) 

 and the septa and external shell were so thin and slight, in comparison to the 

 strength and substance of the siphuncle, it becomes an object of interest to 

 consider what was the condition of the shell in regard to the animal in its 

 living state. 



It was, I believe, first suggested by the late Mr. Miller, "that all chambered 

 shells were inclosed in, or capable of being covered and encompassed by, the 

 animal; for without such a contrivance, that nice degree of buoyancy which 

 they are intended to furnish to the animal, would be destroyed by the adhesion 

 of parasitic testacea*." 



Although we cannot positively pronounce that such was the fact, the pe- 

 culiar thinness of the septa and external shell, described in the specimen of 

 Huronia Portlockii just mentioned, appear to me to require that they should 

 be inclosed and protected. If the animals were contained in the upper 

 chamber only, the lower ones would be exposed to a great extent in length ; 

 and from the size of some of the specimens of Huroniae, we must suppose 

 the animal to have been of considerable weight; and a corresponding strength 

 would be required for the preservation of the shell from destruction if it were 

 external. 



It is true that in Carinaria and Argonauta we have examples of very thin 

 external shells, and in the latter genus, some are of considerable size; but in 

 these instances the animal fills the whole shell; consequently it is not exposed 

 to the same degree of danger of injury as would be a long external appendage 

 to an Orthoceras. In Nautilus the animal inhabits its last chamber only ; 

 and the shell of the earlier chambers is exposed ; but the shell is of consider- 

 able strength, and, being convoluted, every part of its surface is within reach 

 of the soft parts of the animal, which would protect it from the adhesion of 

 parasites. 



Whether all the straight-chambered shells were inclosed or not, and what 

 evidence can be obtained upon the subject, is consequently an object of in- 

 quiry; and in this point of view 1 have paid attention to the Baculite. The 

 external shell of these fossils is very rarely preserved ; but I have met with 

 some specimens which show that it must have been so thin that it could afford 

 to the lower chambers little or no protection from external violence ; and it is 

 difficult to imagine that shells of this form and size would be free from danger 

 if external and thin in substance. 



Wherever I have seen the last chamber of Orthocerata well preserved, I 

 * Geological Transactions, 2nd Series, vol. ii. p. 58. 



