144 GEOLOGICAL INVESTIGATION. 



Such a specimen may show the character and distance 

 of the septa approximately, and give some idea of the 

 curvature, and of the relative size of the chamber of 

 habitation ; but unless we can know the exact relative 

 position of the exhibited plane, then the true amount of 

 taper and curvature, the degree of convexity and marginal 

 arching of the septa, the shape of the septal sutures, the 

 position, size, and shape of the siphon, and the shape of 

 the transverse section of the shell are all unrevealed ; 

 yet unless a fair number of these elements can be made 

 out, little progress can be made in the specific determi- 

 nation of the fossil. In some cases a more or less longi- 

 tudinal section of the siphon appears in the section of 

 the shell. This at once reveals the true plane of the 

 section, and becomes the key to many of the specific 

 characters which we desire to know. 



Under the difficulties mentioned, it has been necessary 

 to adopt many devices for bringing to view the concealed 

 structure. Grinding and polishing the surface has been 

 of much assistance ; but sawing across the fossil at one 

 or more points, followed by a thorough polishing, labo- 

 rious and time-consuming as it is, has often yielded the 

 most satisfactory and decisive results in determination. 

 It is scarcely necessary to mention that the making of 

 microscopic slices has also proved of invaluable service. 



Following the natural division of my subject, I will 

 speak first of the points of progress in the knowledge of 

 the local stratigraphy, and then of those belonging more 

 particularly to paleontology, so far as the latter have 

 not been necessarily brought forward in the stratigraphi- 

 cal discussion. 



In regard to the stratigraphy, one point gained is that 

 it proves to be far more complicated than it was at first 

 supposed to be. 



In place of the one or two folds of the two limestones, 

 at first made out, there now appear to be several main 



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