18 MR. R. D. DARBISHIKE ON GREAT-ORME^S-HEAD 



mistakeable traces of beach-wear — the holes made by 

 burrowing mollusks^ necessarily made while the rock in 

 which they appear was near^ if not below,, low- water mark. 

 It is convenient to designate these holes as those of Pho- 

 lades, which they most nearly resemble. 



There is so endless a variety in the forms in which at- 

 mospheric erosion marks limestone rocks in positions of an- 

 cient exposure^ that under some circumstances it might not 

 be difficult for a geologist unfamiliar with the habit of the 

 living animal either to overlook, as the effect of inorganic 

 destructive processes, what are true Pholas-\io\e^, or, if 

 seeking these, to mistake those for them. A working na- 

 turalist is not so liable to this error ; and, insisting on a 

 perfect symmetry of the hole, and the grouping of several 

 holes in close proximity, he will more readily detect the 

 true molluscan excavation, and distinguish it from struc- 

 tural decay. 



I think I may venture, on the ground of a considerable 

 familiarity with such holes, to assert that there is the 

 greatest probability in the identification of the series of 

 holes I am about to mention. 



Owing, I presume, to the situation of the Head, and the 

 want of an assistant observer, I have found it very difficult 

 to ascertain measurements of elevation. The heights I give 

 are in every case the mean results of at least duplicate ob- 

 servations ; but I cannot speak quite confidently of their 

 exactness. Pholas-h.o\e^ occur frequently at various heights 

 all over the Great Orme^s Head where the limestone is of 

 a sufficiently compact texture, and certain conditions of 

 shelter from atmospheric wear obtain. 



They are found either in the rock (where they are to be 

 sought near the edges and on the undersides of projecting 

 slabs, in situ or fallen), or in larger or smaller detached, 

 rounded, sea-worn blocks of stone, such as lie on or in the 

 sward on many of the grass-grown slopes of the hill. 



