TYPE AMMONITES— VI Feb. 



importance. Hitherto where this has been recorded in the legend at 

 the foot of the plate, it has been placed in the following full form, 

 SI. EL — , Li — , L2 — , per cent, of whorl-breadth or more concisely 

 EL — , Li — , L2 — of — (whorl-breadth understood). Yet it is 

 advisable, for the saving of space, to record for the future in another 

 manner, congruous with that used for giving proportions. Thus the 

 record will be made in four consecutive items : ist. The breadth of the 

 whorl in millimetres, next the length of EL in so much per cent, of 

 such breadth, next that of Li, lastly that of L2. Thus a set of figures 

 preceded by SI. and reading SI. 20, 45, 54, 23 will indicate as follows : 

 The breadth (or height) of the whorl is 20 millimetres, the external lobe 

 (EL) is in length 45 per cent, of that whorl-breadth, the first lateral 

 lobe (Li) is 54 per cent, and the second lateral lobe (L2) is 23 per cent, 

 of the 20 mm. Beginning with this number (Part 56) the records will 

 be so made. When an item cannot be given, a — will be placed in 

 its position. More and more attention to these suture-line details 

 should be urged on those who desire to identify species of Ammonoids. 



The whorl-breadth is given in the same way as it is taken in regard 

 to proportions, because where the suture-line is painted in on the specimen 

 and the figure of the specimen is reproduced with such line in place, 

 that is the easiest method for measurement. But when a suture-line 

 is traced off, and is reproduced with the full curve of a gibbous side 

 extended to a straight line, the necessary allowance must be made, for 

 the difference in length of a base-line from a straightened-out curve 

 beyond a base-line of mere length without the curve. The first base-line 

 should be reduced to the amount of the second, a feat easily accom- 

 plished by means of any of the little wheel-measuring instruments. 



Other proportions may sometimes require to be given, and should 

 be provided for — the Auxiliary lobes and the Internal lobe : they may 

 be marked as A', A" and so on and I, followed by their proportionate 

 figures. 



A further proportion is important in the cases of Stepheocerates, 

 Perisphinctids and Hke forms — the length of what is known as the 

 retracted inner portion, or the suspensive lobe, or, as the Germans 

 concisely call it, the Nahtlobus, that is the seam lobe or the lobe by the 

 whorl- junction. Nahtlobe seems good to adopt, with the symbol N. 

 The proportion of this lobe when given will come as an item marked 

 by a preceding N. 



The length of the Nahtlobe is the length of the whole series of lobes 

 which run down the inner part of the whorl inside Li. See, for instance, 

 Cymatosphindes cymatophorus, CDLb. The length is measured from 

 the bottom of the lateral saddle Si, which, in fig. i of the plate referred 

 to, just touches the bottom line of the figure. From that point a line 

 is drawn towards the inner edge to run parallel with the guide-line. 

 Or the same result can be obtained in another way : — measure from 

 base of ES to the guide-line and then add any overplus below the 

 guide-line or deduct any minus quantity short of it. In the figure 

 referred to there is an overplus of not quite 6 mm., the full length of 

 N is 52 mm. The length of base-hne is 49 mm., so the value of N reads 

 as N 103. 



A good instance of the necessity of attending to the proportions 

 of suture-lines occurs in this work. Three species of Wheatleyites were 



