part 1] JUKASSic chkojtology : lias. 93 



about 150 mm., while a third were characterized by small species, 

 not exceeding 50 mm. and often less. 



Table IX embodies a series of rough estimates of the largest 

 diameters attained by British species of ammonites during the 

 times mentioned, and notes also the hemerse represented by the 

 ammonite faunas in the Gloucestershire strata described by 

 Mr. Richardson, 1 also those of Hierlatz. 





Table IX. — Size 



and Distribution 



PART OF THE LlAS. 



or Ammonites in 









Dates. 

 i 



Size in 

 millimetres 



428 



380 



. Glot 



Localities. 





Wessexiai 



1. 

 Raasayiiii 



7. 



icestershire. 



[x] 



X 



X 

 X 

 X 



X 

 X 



[xj 



X 



[X] 



[x] 



H 



ierlats. 



6. 





150 













4. 





95 





3. 



2 b. 

 2 a. 



bispinigerum 



subplanicosta 



70 



50 



110 



X 



lb. 



la. 



Dei ran 



8. 



Kcliiocerates 



50 



50 



165 



X 



7. 





380 





6. 





450 









135 





4. 





300 





3. 



o 



1. 

 Mercian 

 7 c. 



simpsoui 



Gagaticeras 



200 



50 



50 



X 

 X 



7 b. 





50 



X 



7a. 





200 













Concerning this table, it may be remarked that the figures given- 

 are round numbers, and that all species of 50 mm. or less have been 

 entered as 50. The largest species are not necessarily those which 

 give their names to the hemerse, and some of them are as yet 

 unnamed. That periods of large and small ammonites follow each 

 other like waves (have an undulatory character when plotted) is. 

 shown in Table X (p. 95), which also graphically illustrates that 

 during the period mentioned (Mercian 7 to Wessexian 1) there 

 were only these following dates of big or moderately large am- 

 monites, nearly all the rest being times of fairly small forms 2 : — 



1 Those present, but not described by him, are placed in brackets. 



2 [In order to avoid misconception, perhaps it is necessary to point out 

 that the present is only a plotting record of maximum sizes at certain dates : 

 therefore, all contemporary species below the maximum are disregarded. Quite 

 another investigation would be to plot the sizes of all contemporary species 

 (large, medium, and small), and then to illustrate chronological variations in 

 maxima, minima, average, numerical frequency, and so on. But nothing so ■ 

 elaborate has been given here.l 



