334 Correspondence — Benwyan. 



The little woodcut given in your April number may be, roughly, a 

 fourth or a sixth of their usual scale, and they appeared to be best 

 observable where partly sheltered, as if showing a certain stage or 

 condition of development or depth of weathering, to which greater 

 exposure was unfavourable, causing the removal of whole patches of 

 the tracery. 



The material seemed always to be ordinary mortar, composed 

 of lime, sand, etc. ; and upon old walls that had been " puttied," 

 i.e., covered with a thin layer of smoother, and more calcareous, 

 material, they were specially well preserved, the harder laminae 

 relieved, so that the original surface would have served to print 

 them from ; they also occurred in rough plastering and in very old 

 ruins, as well as much more modern structures, on damp-looking 

 internal walls of churches, and such like situations ; an almost 

 invariable condition of their existence being some sort of plane- 

 surface, more usually vertical than horizontal, originally given to the 

 composition. 



As a consequence of this last observation, probably originated 

 the idea which I have often heard advanced, that the marks were 

 due to the rotary motion of the plasterer's arm causing a mechanical 

 distribution of coarser and finer particles of the plaster, but this is 

 evidently not the case, the old marks of the plastering tool being 

 sometimes seen sweeping in broad curves across the concretionary 

 pattern. 



The structure appears to result from segregation or crystallization, 

 or a combination of both, set up among the silicious and calcareous 

 materials of the plaster. 



The concentric character of the pattern is frequently quite as 

 perfect as any other concretionary structure, but has much less 

 tendency to interruption by breaks and shifts than is to be observed 

 in agates, etc., of which numerous examples occur in the beautiful 

 plates adorning Mr. Euskin's contributions to former numbers of 

 your Journal. 



It seems strange that I can hardly recall an instance of the occur- 

 rence of these markings on the " Chimam " walls of India, and I 

 have never seen them, except in materials of which lime formed a 

 considerable part, nor could I detect anything like signs of com- 

 mencement, termination, or progressive production of the structure. 



^^L^^^i ^i^yj^^' Benwyan. 



3vnisc:E]XjXj.A_isrEOTJS. 



Stjpplementaky List of Type Specimens of Fossil Fishes in 

 THE British Museum. — The following additions to the Type 

 Specimens of Fossil Fishes in the British Museum were purchased 

 after the catalogue published in the May number of the Geol. Mag., 



