492 THE LOWER LUDLOW FORMATION. [May I9OO. 



mudstone was approached, but their more arenaceous character was 

 equally well marked. 



The Rev. J. F. Blake said that he was convinced of the extreme 

 importance of the subdivision of the great unvarying masses of the 

 Lower Ludlow, particularly in the typical districts ; but he could 

 not help being somewhat sceptical as to the naming of graptolites in 

 general. They preserved so few characters, and these appeared to 

 be so liable to be masked by accidents of preservation, while the 

 writers on the group did not seem to have made up their minds as 

 to the relative value of the characters. 



Prof. Watts congratulated the Authoress on the good work done. 

 As he had previously worked in one of the districts alluded to, the 

 Long Mountain, he believed that the Authoress was right in her 

 classification. He drew attention to the admirable illustrations 

 which accompanied the paper. 



Mr. C. D. Sherborn also spoke. 



Prof. Lapworth pointed out the great interest of this paper, as 

 showing for the first time that the principle of zonal mapping by 

 means of graptolites is applicable even in the very latest formation in 

 which the Rhabdophora have hitherto been certainly recognized ; 

 and, further, that this zonal method of stratigraphy not only enables 

 us to correct the previously published maps of the Silurian rocks 

 of the Welsh Borderland, but affords us a means of mapping the 

 monotonous flaggy and mudstone series of all Eastern Wales in 

 detail in the future, and of bringing them into direct comparison 

 with the corresponding strata of other districts. The main obstacle 

 which had formerly stood in the way of the graptolithologist who 

 attempted the study of these Lower Ludlow graptolites, was the 

 uncertainty of the identity of many of our British forms with the 

 Bohemian species originally described in the classical works of 

 Barrande. Thanks to the detailed revision of the Bohemian graptolites 

 recently published by Dr. Perner, and to the fact that the Authoress 

 and Miss Elles, previous to the commencement of their investiga- 

 tions, paid a visit to Bohemia, examined Barrande's types in the 

 Prague Museum, and, guided by Dr. Perner, collected many of the 

 critical forms in the field, we have now a guarantee that the identi- 

 fications may be regarded as reliable. Mr. Hopkinsons work among 

 the Ludlow graptolites had long been recognized as the most im- 

 portant hitherto accomplished among these rocks. The Authoress 

 had found it of the highest value in her own work, and had described 

 and figured specimens of the Rhabdophora from his collection among 

 her types. Too little is as yet known of the Cladophora to make 

 them available lor zonal purposes. 



As respects Prof. Blake's observations on the ammonites and 

 graptolites, it was but natural that the students of the one group 

 should find themselves unable to appreciate the minute distinctions 

 relied upon by the students of the other, or the necessity for the very 

 detailed classifications insisted upon. But the specific forms in both 

 groups are certainly of high stratigraphical value, and no doubt all 

 disputes and difficulties will disappear in the course of time. 



