102 



THE GEOLOGIST. 



quently rounded into large foliation as we see in A. 

 Calypso, Guettardif Thetys, Veiledce, Alpinus, ^c. 



Causes of error in the study of the Septa.—The lobes and 

 saddles of the Septa being formed by the edges of the mantle, 

 and attached to the internal sides of the shell, they are 

 covered and entirely concealed, whilst the shell exists, and 

 their appearance on the exterior is occasioned either by the 

 alteration of a portion or the disappearance of the entire 

 shell. Whenever they are visible, we may rest assured that 

 we are examining an internal cast, and not a perfect shell ; 

 and that therefore the external appendages are diminished. 

 The outline of the Septa are apparent in the cast on the 

 foreign matter, which filled all the cavities, when the shell 

 was still entire, and took the impression of all the parts. As 

 the foliations of the Septa are directly in contact with the 

 shell, it follows that they become more divided or multiplied, 

 as they approach the external parts, since the centre of 

 the Septum is smooth, and the trunks of the main divisions 

 separate from this point, and ramify towards the circumfer- 

 ence. I wish to draw particular attention to this disposition^ 

 which might lead a person who was not aware of it into 

 error. If the cast is in good condition, and has suffered no 

 alteration, the divisions of the Septa will be presented in all 

 their details ; but if on the contrary, any portion of the ex- 

 terior of the cast has been removed by abrasion, then a 

 great number of the details disappear, and with the same 

 general disposition, we find Septa essentially different from 

 what they ought to be ; the dissimilarity of the Septa aug- 

 ments with the progress of the external alteration, until it 

 becomes complete. Thus, when examining the Septa, it is 

 always indispensable to assure ourselves, that no alteration 

 has taken place in the external parts, before we can pro- 

 nounce on their true form. 



