part 1] ON THE MORPHOLOGY OF THE AMMOJs^ITE SEPTUM. 27 



Tliere has also been a limited discussion upon the mechanical 

 conditions which have heen concerned in modelling the septum, 

 and upon the relatiSns of newh^-formed septa to their predecessors. 



II. METH0I3S OF I^^YESTIGATION. 



The chief obstacle to the study of the sej^tum is the difficulty 

 experienced in isolating the casts of successive chambers in such a 

 way as to show a septal face with all its marginal frilling complete. 

 This difficulty increases as the frilling becomes more complex. 

 It is very seldom that a specimen breaks naturalh^ in such a way 

 as to show a complete septum, for the minor, if not the major,, 

 frills are almost invariably broken. 



The septa shown in PI. Ill, figs. 1, 6, & 7, were cleaned with 

 acid and tools, and supplied much useful information. The 

 alternative method was to file away the superficial portions of a 

 whorl in successive laj'^ers. This brought to view a series of sec- 

 tions of the septa cut approximately parallel to the periphery. 

 Such sections are hereinafter referred to as septal sections. 



To secure accurac}^ in working, much use was made 'of the 

 instrument figured in PL II, fig. 1, and shown in the accompanying 

 text-figures (1 & 2, pp. 28-29) in plan and elevation. 



This was designed with the expert assistance of Dr. P. E. Shaw, 

 of the Physics Department of University College, Nottingham, 

 ^t consisted of two parts. The one was a microscope with micro- 

 meter e3^epiece and graduated rack-scale, of the kind commonly 

 used in physical laboratories. The other consisted of a graduated 

 turn-table (fig. 2 a) mounted on another table (b) which could 

 travel horizontally. This latter had one edge provided with a 

 vernier and laid against a fixed horizontal scale. Above the table 

 was a double-pointed needle (c) held up in the position of rest 

 shown in PI. II, fig. 1, by a lever (d) actua^ted by a spring (e). 

 The needle 2Jassed loosely through the lever, and rested on it by 

 means of a collar (f). When the lever Avas pressed down the 

 needle was left free to fall. As it was very light, a spring (y) was 

 insei-ted which, b}^ pressure on a collar (Ji) near the lower end, 

 ensured a certain but gentle contact of the j^oint with the surface 

 of 'the table, or of the object of stud}^ When the lever was 

 released, the needle was at once lifted b}^ it to the position of rest. 

 The luicroscope was focussed upon the upper point of the needle, 

 and its precise position, as recorded in the micrometer eyepiece 

 and on the rack-scale, was noted. The amount of drop of the 

 needle on to the table, or on to different points of the surface of 

 an object, could then be easity registered — for small movements by 

 means of the eyepiece alone ; for large ones, by the rack-scale. 



In obtaining the measurements for producing the contoured 

 plan of a septum (fig. 4, p. 32) the specimen was securely fastened, 

 with the septal face upwards, on the centre of the turn-table. This 

 being caused to travel horizontally, a series of heights, or depths, 

 was taken at definite intervals along one diameter of the septum. 



