BELLEROPHON. 



are occasionally exposed without the shell, this has been 

 shown even in the Nautilus pompilius ; it will be no answer 

 to this fact, as a reason that the Ocythoe is a parasite, to 

 say that the Nautilus is a strong shell, and the Argonaut a 

 very thin testaceous covering', because the same fact may 

 be observed in the Carinaria, whose shell is even thinner 

 than that of the Argonaut. It must also be kept in mind 

 that the Ocythoe has the inferior part of its body covered 

 with as thick a mantle as the superior. 



Fifthly. In such of the^ Cephalopoda as possess any- 

 thing resembling a shell, it is entirely internal and not 

 placed outside the mantle, nor is it produced by testaceous 

 secretion from that organ. 



It is to be observed that in those cases where the 

 shell is such as Aplysia, Builaea, &c. it is really outside the 

 mantle, being covered more or less completely by the edge 

 of the mantle folding over it. 



We conclude, therefore, for the above reasons that the 

 Ocythoe usually found in the Argonaut is not the animal 

 by which it has been formed, and moreover that the animal 

 which forms the x4rgonaut is not a Cephalopod, for it is not 

 affixed to the inside of the Argonaut by any muscle ; the 

 Argonaut has an external epidermis, wherefore there can 

 be no doubt that its animal is also affixed to the shell by a 

 muscle, which the Ocythoe cannot be, 



Bellerophon has been described as a chambered shell, 

 with a Siphon (like Nautilus); this has, however, been 

 proved by Defrance to be a great mistake; it really has 

 nothing which can be compared to chambers or a siphun- 

 cle. It is a spiral, involute, thick shell, umbilicated on 

 both sides and very nearly symmetrical; its aperture is 

 very large, semilunate ; the back of its shell is rounded, 

 obtusely bicarinated, and there is a notch in the lip between 

 the keels. 



Only known in a fossil state and characteristic of the 

 carboniferous Limestone, and the oldest secondary strata ; 

 in these it is frequently found changed to Silex. 



