KENNAED & WOODWARD : ON BRITISH TBUNCATELLINA. 295 



Britannica, Cylindrica. 



Margin of the outer lip in line and is not in a line with 



with the outline of the the outline of the 



cylindrical portion of the cylindrical portion of the 



shell. shell. 



Oral Characteristically three den- None. 



armature. tides. 



Columellar lamella strong, 



deep - seated, constantly 



present in mature specimens, 



and visible in oblique view. 

 Parietal lamella, one short, 



obscure tubercle, visible in 



obhque view, but not always 



present. 

 Palatal fold an immersed, 



rounded or oblong tubercle, 



visible in front view, but not 



always present. 



The denticles, or rather tubercles, that when developed form 

 a prominent feature in adult T. britannica, seemingly only develop 

 late in life. The columellar tubercle, which is the most conspicuous 

 as a rule, appears to form first, when the shell has come to full 

 growth. The palatal tubercle develops next, and later the parietal 

 (though we have seen a specimen with parietal but no palatal 

 tubercle) completing the typical three in the old age of the snail. 



Thus in a considerable series, some forty or more, collected in 

 April or May at Portland by Mr. J. E. Cooper, whilst the columellar 

 tubercle was present in the full-grown examples, only five of the 

 number showed the palatal one, and none the parietal. 



On the other hand, out of thirteen collected at Portland by 

 Mr. A. E. Salisbury in August, seven showed the palatal tubercle, 

 none the parietal. Of two batches, totalling twenty-seven, from 

 the same locality (time of collecting not ascertained) in Mr. Tomlin's 

 collection, leaving out of account three immature and one 

 decomposing specimen, all showed the columellar tubercle, ten the 

 palatal, and three the parietal. 



In the Swanage specimens collected by Mr. Tomlin in May and 

 June of two succeeding years, the columellar tubercle is placed much 

 further back than in those from Portland and cannot be detected by 

 a simple pocket lens; their presence is, however, easily established 

 under the microscope. Out of thirty-seven examples (including those 

 already presented to us) six were immature, all the rest showed the 

 columellar tubercle, while only four exhibited the palatal and but 

 two the parietal. 



Of three large specimens from Lyme Eegis, shown us by 

 Mr. Hartley, only one, the smallest, showed the columellar denticle, 

 placed very far back, and none of them the other denticles. 



Individuals exhibiting all three denticles would, therefore, seem 

 to be scarce. 



