53 



ON THE 

 PERIOSTRACUM OF HELIX ARBUSTORUM, Mull. 



By G. SHERRIFF TYE, 



Birmingham. 



On cleaning the shells of this species preparatory to putting them in 

 my cabinet, I had often noticed that on the ultimate whorl for the 

 space covering the last period of growth and completion of the 

 shell, there appeared a dark fuscous covering which was readily 

 removed by a little vigorous rubbing with a damp cloth, leaving 

 the shell cleaner looking, and with its periostracum apparently 

 undisturbed. 



Last year I collected this species in plenty at Buxton and 

 Castleton, in Derbyshire, where it occurs generally distributed about 

 the district with various varieties, flavescens being fairly abundant. 

 My attention was thus more particularly called to the peculiarity 

 spoken of above. Careful examination revealed the fact that the 

 periostracum is double. This doubling starts from the commence- 

 ment of a period of growth — where it is scarcely perceptible — and 

 goes on gradually increasing in strength until the finish of that 

 growth, being most easily seen in the last, or finishing part. 



The outer periostracum of early life is probably rubbed off by the 

 creature's movements among damp vegetation, as it is not so firmly 

 fixed to the inner — or true — periostracum below it, as that is to the 

 shell. In patriarchal individuals it is not of such extent as in the 

 more recently finished shells, although I have never seen a shell 

 whereon it did not show plainly round and about the lip. 



I have forwarded to Mr. W. E. Collinge five numbered shells in 

 illustration of my remarks, the first four being from Buxton. 



No. i. The outer layer of periostracum is removed in a band 



(going towards the mouth) to a line terminating below the 



peripheral band. The inner periostracum is removed (exposing 



the shell) in a line above the band, thus showing the two 



periostracal layers. 

 No. 2 has the outer covering only, removed in a curve terminating 



at the mouth above the band, but not below it. 

 No. 3 (var. flavescens) is treated in a similar manner to No. i, and 



shows the three bands, shell white, lower periostracum lemon, 



upper ochre. 



